Why Should We Care About Crop Factor?

[Ysarex]

.....The only reason that the DOF varies is because people generally tend to get closer to the subject when they are using a larger sensor.....

That's just flat-out wrong. Go back and read through the thread.

Joe

 

[Ysarex]

>post 32>>>Think of it this way: Again assume a standard scene -- this time a landscape with an horizon at infinity. Maximum DOF advantage will occur with the lens set at the hyperfocal distance for the f/stop used. To maintain the exact same content in the photo each reduction of the media size will require a shorter lens focal length and at the same f/stop the hyperfocal distance will keep moving closer to the camera. Hyperfocal distance is calculated based only on lens focal length, f/stop and CoC. The media size is a factor because it forces the use of a different lens focal length to maintain the same content. The result again is more DOF in practice using the smaller media.> SNIP>
Joe

See, the process Joe described above was EXACTLY the reason Kodak invented the tiny "disc format" snapshot/purse/carefree film camera format back in the 1970's. Yes, Kodak invented a whole NEW format size as a way to get unlimited depth of field pictures for snapshooters! Using bigger film formats, like 126 cartridge, the minimum focus distance of the cameras at snapshot f/stop was around 4 feet, and people often shot too close, and got out of focus images. Same with 110 Instamatics with fixed focus lenses-those had focus in to about 3 feet, which was too long of a minimum focus distance! TOo many OOF snapshots!

WHhen people photographed babies, or flowers, or close-ups of whatever using their 126 or 110 cartridge load cameras, there were MANY OOF images. Kodak realized, they needed to invent a super-SMALL format, and the Disc Format was born. Focused at 3 feet, with its ultra-short lens (I think it was 3.5mm or 3.7mm) and teeny-tiny Point and Shoot Digital 1/1.7" (ie-more or less what we now think of as small-sensor digicam sized), the fixed-focus lens was set at 3 feet, which was the hyperfocal focusing distance for that lens and format at the fixed f/stop, and BOOM!

FOCUS-FREE photography was achieved with the Kodak Disc Camera format--based ENTIRELY on "optics". The laws of optics. With that small a format and that short of a normal lens, there was NO need for focusing the lens...anything that the camera was aimed at would pretty much be in good focus. (This is basically the recounting of Modern Photography's Herbert Keppler's account of how Kodak engineers invented the Disc format--to solve an actual problem by REDUCING format size and going to an ULTRA-short focal length standard lens.)

I just kept wanting to put the bleep things into and View-Master and it never did work.

Joe

 

[Derrel]

I just kept wanting to put the bleep things into and View-Master and it never did work.

Joe

Funny! Odd thing...when I was in my early 20's, I used to work for a camera store near Beaverton, Oregon. About 1/4 mile away was the View-Master factory. Anyway,since View-Masters were made here locally, we had a few regular customers who were really huuuuge fans of stereo photography, and one older gentleman in particular worked at the View-Master plant and occasionally he would bring in View-Masters and custom discs he had shot AND HAD MADE AT WORK! He used old Stereo Realist cameras.

It was soooo cool to see Columbia Gorge waterfall shots, and Rose Parade floats, and just local, Oregon coast shots and Willamette Valley shots all done by a really skilled shooter, and made in the current era!

Anyway, the View-Master process was invented right here in town in 1939 by Bill Gruber, a Portland photographer. So, every time I see a View-Master at Goodwill, I think back not to the ones I had as a little kid, but to the discs I saw made 20+ years later by a fellow from Beaverton.View-Master factory supply well - Wikipedia, the free encyclopedia

But yeah, I think Kodak's engineers might have thought about the View-Master disc system, but then again, there WAS an old-timey revolver-like camera that used a revolving disc film system, wayyyyy back in the 1910's I believe.

 

[Ysarex]

I just kept wanting to put the bleep things into and View-Master and it never did work.

Joe

Funny! Odd thing...when I was in my early 20's, I used to work for a camera store near Beaverton, Oregon. About 1/4 mile away was the View-Master factory. Anyway,since View-Masters were made here locally, we had a few regular customers who were really huuuuge fans of stereo photography, and one older gentleman in particular worked at the View-Master plant and occasionally he would bring in View-Masters and custom discs he had shot AND HAD MADE AT WORK! He used old Stereo Realist cameras.

It was soooo cool to see Columbia Gorge waterfall shots, and Rose Parade floats, and just local, Oregon coast shots and Willamette Valley shots all done by a really skilled shooter, and made in the current era!

Anyway, the View-Master process was invented right here in town in 1939 by Bill Gruber, a Portland photographer. So, every time I see a View-Master at Goodwill, I think back not to the ones I had as a little kid, but to the discs I saw made 20+ years later by a fellow from Beaverton.View-Master factory supply well - Wikipedia, the free encyclopedia

But yeah, I think Kodak's engineers might have thought about the View-Master disc system, but then again, there WAS an old-timey revolver-like camera that used a revolving disc film system, wayyyyy back in the 1910's I believe.

Oh I'm jealous! I loved View-Master. I used to have one and it went to Goodwill. Now that I'm retired I just might see about picking one up again. I could go spend my retirement account on ebay buying disks.

Joe

 

[Ysarex]

DOF has nothing to do with the size of the sensor. The only reason people think it does is because you'd need to get closer to the subject with a full frame sensor to fill the frame. It's the different subject distance that changes the DOF, not the actual size of the sensor.

Well, you're wrong about that.

DOF depends upon 4 variables:
- Focal length
- Distance to subject
- Current Aperture
- Sensor resolution (or resolution of your print)

And what exactly focal length means depends upon sensor size. Thus DOF depends upon sensor size.

Wait... what? You're saying the focal length of my lens changes if I remove it from my D800 and place it on my D7000? That's interesting...

The focal length of a lens is always the same (unless it's a zoom lens, but then it is always the same range). The only thing changing with a different sensor size is the field of view, the physics don't magically change.
The only reason that the DOF varies is because people generally tend to get closer to the subject when they are using a larger sensor.

Crop factor is great when you are using a long lens and could use a little more. Crop factor sucks when you are using a wide angle and could use a little less.

My D800 has a more or less similar pixel density on its sensor as my D7000. This means that if I crop my D800 image in Photoshop with the same factor as a crop sensor I get more or less the same image. It's not the fact that the sensor is smaller that you can zoom in more, it's the pixel density that does the trick. Given the same density, the size of the sensor doesn't matter at all.

Do this:

Put a tripod on the ground in front of an immoveable subject.
Put the D800 along with an FX zoom on the tripod.
Frame your photo and note the framing carefully.
Focus on the subject.
Take the photo and remember the f/stop.
Put the D7000 on the tripod and do not move the tripod so much as a mm.
Take the zoom lens off the D800 and put it on the D7000.
Zoom the lens to frame the exact same photo you just took with the D800.
Focus on the subject (same focus distance as previous photo).
Take the photo using the same f/stop.

The photo taken with the D7000 has more DOF. If you don't think that's the case then you need to adjust you're thinking.

Joe

 

[Judobreaker]

Yes of course it differs, I don't even have to test this. You're changing the lens physics to a different length.
It's still not the sensor that gives you a smaller DOF in that case, it's the different focal length of the lens.

My point is that the size of the sensor itself has no effect on DOF whatsoever.
Try this:

Put a tripod on the ground in front of an immoveable subject.
Put the D800 along with an FX fixed lens on the tripod.
Frame your photo.
Focus on the subject.
Take the photo and remember the f/stop.
Put the D7000 on the tripod and do not move the tripod so much as a mm.
Take the lens off the D800 and put it on the D7000.
Focus on the subject (same focus distance as previous photo).
Take the photo using the same f/stop.
Load the D800 image in Photoshop (or any image processing software for that matter).
Crop the image with a factor of 0.625 (1.6x in reverse, to get from FX to DX format).

And now tell me if the DOF is still thinner.

 

[Ysarex]

Yes of course it differs, I don't even have to test this. You're changing the lens physics to a different length.
It's still not the sensor that gives you a smaller DOF in that case, it's the different focal length of the lens.

My point is that the size of the sensor itself has no effect on DOF whatsoever.
Try this:

Put a tripod on the ground in front of an immoveable subject.
Put the D800 along with an FX fixed lens on the tripod.
Frame your photo.
Focus on the subject.
Take the photo and remember the f/stop.
Put the D7000 on the tripod and do not move the tripod so much as a mm.
Take the lens off the D800 and put it on the D7000.
Focus on the subject (same focus distance as previous photo).
Take the photo using the same f/stop.
Load the D800 image in Photoshop (or any image processing software for that matter).
Crop the image with a factor of 0.625 (1.6x in reverse, to get from FX to DX format).

And now tell me if the DOF is still thinner.

And you're taking two different photos. If you read through the thread the point was stressed over and over that if you take THE SAME PHOTO with two different size sensors the smaller sensor photo will have more DOF. No one ever said it was a direct function of the sensor. In fact that was also made clear; the relationship is indirect but it is no less real. It is correct to say that in general when you use smaller sensor cameras you get more DOF. And it is most certainly not because people with larger sensor cameras get closer to their subject.

Joe

EDIT: Sorry, I didn't carefully read through your list. If you're going to crop the D800 sensor down then it's not a D800 any more it's just a D7000 in which case a D7000 has the same DOF as a D7000 -- duh. You have to use each camera's full sensor for the photo otherwise you're not comparing different size sensors are you.

 

[Judobreaker]

And you're taking two different photos. If you read through the thread the point was stressed over and over that if you take THE SAME PHOTO with two different size sensors the smaller sensor photo will have more DOF. No one ever said it was a direct function of the sensor. In fact that was also made clear; the relationship is indirect but it is no less real. It is correct to say that in general when you use smaller sensor cameras you get more DOF. And it is most certainly not because people with larger sensor cameras get closer to their subject.

Of course it is because people with a larger sensor get closer to their subject... That is the only explanation when we're assuming you use the same lens.
Either you get closer or you use a longer focal length, but in both cases it is not the sensor size that changes the DOF... That is simply impossible.

EDIT: Sorry, I didn't carefully read through your list. If you're going to crop the D800 sensor down then it's not a D800 any more it's just a D7000 in which case a D7000 has the same DOF as a D7000 -- duh. You have to use each camera's full sensor for the photo otherwise you're not comparing different size sensors are you.

I'm cropping a photo made with the sensor of a D800. If the cropped D800 and the uncropped D7000 are exactly the same than that proves sensor size doesn't change DOF right? Unless in some magical way I can change my DOF by cropping a photo in Photoshop...

 

[amolitor]

The exact same argument can be made to prove that nothing at all changes DoF.

 

[Ysarex]

And you're taking two different photos. If you read through the thread the point was stressed over and over that if you take THE SAME PHOTO with two different size sensors the smaller sensor photo will have more DOF. No one ever said it was a direct function of the sensor. In fact that was also made clear; the relationship is indirect but it is no less real. It is correct to say that in general when you use smaller sensor cameras you get more DOF. And it is most certainly not because people with larger sensor cameras get closer to their subject.

Of course it is because people with a larger sensor get closer to their subject... That is the only explanation when we're assuming you use the same lens..

You are assuming the same lens. We are not. We have all along been assuming that we are using the two different sensor cameras to take the same photograph. We made that very very clear. When I first corrected you I said, "Assumption: You want to take the exact same photo with two different cameras that have different size sensors." In order to do that the two cameras must be placed in the same physical location. We know what we have been talking about all along. Read the thread.

Either you get closer or you use a longer focal length, but in both cases it is not the sensor size that changes the DOF... That is simply impossible.

And no one in this thread has ever said that or made that claim -- at least I haven't.

Again in the post where I first corrected you I said, "Now it's not that DOF is a function of sensor size directly. It's an indirect effect but no less real. It's this: In simplest form DOF = f/stop + magnification. When you do the math on magnification and factor in that you're using the cameras to take the exact same photo, you must get more DOF from smaller sensor cameras and less DOF from larger sensor cameras."

When I corrected you then I clearly affirmed the working assumption: TAKE THE SAME PHOTO.

With both cameras in general use the camera with the smaller sensor must produce photos with more DOF. That is because the smaller sensor plays a role in setting the practical limits of magnification. When the same photo is taken with two different cameras using different size recording media the photo from the camera with the smaller media will have more DOF. That's the claim that I have made. I have made no other claim. That claim is correct.

AND going all the way back to the OP's original post he gives the example of crop factors being used to compare focal length lenses between formats. AND THAT'S BECAUSE when people use different format size cameras they change lenses!! They do that in order to be able to TAKE THE SAME PHOTOS! A 50mm is commonly used with crop sensors to take portraits. Photographers don't put a 50mm on a FF sensor and get closer to take a portrait. They stay in the same place and use an 85mm!! And one of the big reasons they chose that FF camera for portraits in the first place is because they want the reduction in DOF that comes with using a larger sensor!!!

Joe

 

[Judobreaker]

Your first post in this topic.

Yep, I agree it's way past time to put the whole 1.6 times and focal length equivalent nonsense away. There are however real differences in the kinds of photos you can take one format size to another and it's valuable to be aware of those differences. For example there are real differences in how DOF is going to work as you increase or decrease the size of the sensor.

Joe

 

[Derrel]

Your first post in this topic.

Yep, I agree it's way past time to put the whole 1.6 times and focal length equivalent nonsense away. There are however real differences in the kinds of photos you can take one format size to another and it's valuable to be aware of those differences. For example there are real differences in how DOF is going to work as you increase or decrease the size of the sensor.

Joe

Not to speak for Joe, or in Joe's defense, because he's a big boy and has forgotten more about photography than many people here have managed to learn, but YEAH, his first post pretty much sums it up. There are very real differences in how much depth of field a person has available to him, based on how large the film format, or sensor format, happens to be. Having shot 4x5 sheet film, 6x9 cm, 6x6 and 6x4.5 cm rollfilm, 135 size film AKA "35 millimeter fullframe" film and digital, as well as 1.5x Nikon and 1.6x Canon d-slr, plus 1/2.3" [sic] digital P&S, and diminutive iPhone, I'm pretty damned familiar with HOW depth of field is VERY limited on 4x5 unless camera movements are used, becomes less-limiting on medium format, is not that big an issue on 645, and on 35mm full-frame is easy to work with, and how on 1.5x and 1.6x and smaller, depth of field becomes basically TOO GREAT for the kinds of pictures I want to make much of the time.

If one does not KNOW,as in KNOW intimately and without doubt or confusion, from ACTUAL, you know, hands-on experience, that a SMALLER film size, or a SMALLER digital sensor, gives more depth of field in the same "circumstances" than does a larger-format camera, it might be advisable to just sit back and not make any foolish statements about how there's no difference in DOF behavior between cameras of different format sizes.

Anyway, earlier this AM I made some screen caps. Just wanted to show how foolish some people look. Here are the cold, hard numbers, in which we use two camera formats, and the same lens focal length, an 85mm lens, to frame the SAME EXACT scene. Now, imagine this is at a wedding, in a crowded reception area. With one camera, you stand 20 feet or so away...with the 1.6x Canon, you are forced to stand over 30 feet distant.







See how this shakes out in the REAL world????

 

[Ysarex]

Your first post in this topic.

Yep, I agree it's way past time to put the whole 1.6 times and focal length equivalent nonsense away. There are however real differences in the kinds of photos you can take one format size to another and it's valuable to be aware of those differences. For example there are real differences in how DOF is going to work as you increase or decrease the size of the sensor.

Joe

Yep. I was correct then and I remain correct now. I said there are real differences in how DOF is going to work with changes in the size of the sensor. That's a correct statement though not very detailed. I DID NOT say that DOF is a direct function of sensor size. I did later clarify that sensor size indirectly effects DOF and I'm right. It does. One of the reasons photographers purchase and use larger sensor cameras is so they can take advantage of the ability they get from those cameras to better control selective focus (DOF). You say you have a D800 and you're not aware of that?!! One of the persistent complaints that you hear from people with smaller sensor cameras is that they can't get the same degree of selective focus (DOF) that they see from FF cameras. They can't, and guess why? You're going to say because they're standing too far back and they should move forward like people who use FF cameras do?

So how many times do you need to have this repeated: Take the same photo (same f/stop) with two cameras that have different size sensors and the photo taken by the camera with the smaller sensor will have more DOF. And that is a real difference in how DOF works as you increase or decrease the size of the sensor. Go to DOFMaster and run the simulations if you need more help.

Joe

 

[Ysarex]

Not to speak for Joe, or in Joe's defense, because he's a big boy and has forgotten more about photography than many people here have managed to learn, but YEAH, his first post pretty much sums it up. There are very real differences in how much depth of field a person has available to him, based on how large the film format, or sensor format, happens to be. Having shot 4x5 sheet film, 6x9 cm, 6x6 and 6x4.5 cm rollfilm, 135 size film AKA "35 millimeter fullframe" film and digital, as well as 1.5x Nikon and 1.6x Canon d-slr, plus 1/2.3" [sic] digital P&S, and diminutive iPhone, I'm pretty damned familiar with HOW depth of field is VERY limited on 4x5 unless camera movements are used, becomes less-limiting on medium format, is not that big an issue on 645, and on 35mm full-frame is easy to work with, and how on 1.5x and 1.6x and smaller, depth of field becomes basically TOO GREAT for the kinds of pictures I want to make much of the time.

Hmmm... Derrel speaking for me? I'll have to think about that.

Yeah I used to shoot portraits with 4x5 sheet. f/11 on a head and shoulders wouldn't keep you sharp from the tip of their nose to their eyes yet alone back to an ear. Now I take a head and shoulders of my wife with my little compact and f/4.5 still has the bookcase behind her in focus.

Joe

 

[rambler]

Suppose you set up a 4/3 next to a full frame camera and took the shot using the same exposure equivalent focal distance for example 50mm on the full frame would be 100mm on the 4/3. I believe you would see more included in the full frame because the edges would be further apart than on the 4/3. But, you could achieve the same look with the 4/3 just by backing up several feet. The term "cropped" is just to make it a bit easier to visualize the difference between the two original shots. It just looks as though one must have "cropped" off the edges of the full frame result. However, nothing has actually been cropped. You are just looking at the results of two different sized sensors. Hmm. could someone here actually do this?

Take a look here:
http://www.kenrockwell.com/tech/crop-factor.htm