Why Should We Care About Crop Factor?

[KmH]

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

Joe, now I am confused even more. How the size of the medium alone is responsible for DoF. I thought only lens (and maybe tilt and swing of it) is responsible for DoF, not the size of the negative. Or sensor for that matter. Size of the negative cannot change the size of COC with given lens. Or I am wrong on this one ?

Judobreaker explained it. The point of focus distance changes when you use a crop sensor to accommodate the apparently cropped FoV sensor delivers.

Point of focus distance affects DoF.

 

[sleist]

Joe, now I am confused even more. How the size of the medium alone is responsible for DoF.

What I think he means to say is that the sensor size impacts the depth of field capabilities of a given lens. It is not only factor though.
The smaller the sensor, the larger the minimum possible depth of field will be regardless of the lens used. A 50mm f/1.4 lens at f1.4 will have a smaller depth of field on a large sensor than the same 50mm f/1.4 at f1.4 on a smaller sensor. Even if you used a 35mm f/1.4 on the DX or a 24mm f/1.4 on m43 in order to better approximate a similar field of view, you would still have a larger minimum DOF on a smaller sensor.

Ultimately, what Steve5D states is the truth - find what works for you and use it.

 

[timor]

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

Joe, now I am confused even more. How the size of the medium alone is responsible for DoF. I thought only lens (and maybe tilt and swing of it) is responsible for DoF, not the size of the negative. Or sensor for that matter. Size of the negative cannot change the size of COC with given lens. Or I am wrong on this one ?

Judobreaker explained it. The point of focus distance changes when you use a crop sensor to accommodate the apparently cropped FoV sensor delivers.

Point of focus distance affects DoF.

Please, give me the link. I don't understand how the distance from the subject changes, when using crop sensor.

 

[timor]

What I think he means to say is that the sensor size impacts the depth of field capabilities of a given lens. It is not only factor though.
The smaller the sensor, the larger the minimum possible depth of field will be regardless of the lens used. A 50mm f/1.4 lens at f1.4 will have a smaller depth of field on a large sensor than the same 50mm f/1.4 at f1.4 on a smaller sensor. Even if you used a 35mm f/1.4 on the DX or a 24mm f/1.4 on m43 in order to better approximate a similar field of view, you would still have a larger minimum DOF on a smaller sensor.

Thank you Sleist, that's the mystery I have to understand, how given lens is starting to create smaller CoC, when the size of the sensor or negative is smaller.

 

[JTPhotography]

I think this issue is getting overly confused.

If I set up a tripod and take a photo with a crop sensor camera and 50mm lens, then, keeping the tripod in place and using the same settings, swap the crop body with a full frame body and take the same exact photo, the ONLY difference in the two photos is that there would be less stuff showing around the edge of the photo taken with the crop body. DOF is the same, distortion is the same, the lens is projecting the same image onto the two sensors. If you take the FF body and move the camera and tripod closer and try to recreate the exact image taken on the crop body, your DOF would change because you have moved the camera in relation to the objects in the photo. Make sense?

 

[Derrel]

There **is a significant, noticeable and a very, real difference** between shooting with an APS-C or other smaller-sensor digital camera, and a 35mm-sized or 24mm x 36mm sensor camera( called FX by Nikon, or FF, AKA full-frame digital by many) as well as between a 35mm sensor size camera, and a medium-format rollfilm type camera, like say a 6x6 square camera like a Hasselblad. Mamiya, or Rollei,etc..

The simplest explanation is that the smaller the sensor or film size, the MORE depth of field there is at each picture "angle". Let's say we want to make a normal-lens picture--not an ultra-telephoto, tightly frames image, and not an ultra wide-angle panoramic image, just a normal picture. A point and shoot with a teeny-tiny sensor has almost infinite depth of field with its normal lens; an 8x10 view camera has very little depth of field with its normal lens; in between those extremes, there is MORE depth of field at each picture angle of view, as the sensor or film size gets smaller.

Here is one of the best articles I've seen on the issue. One that's accurate, and factual, unlike many pieces written about this issue.
Depth of Field and the Small-Sensor Digital Cameras - photo.net

SOME BULLET POINTS:

1. For an equivalent field of view, the small-sensor camera has at least 1.6x MOREdepth of field than a full-frame camera would have - when the focus distance is significantly less then the hyperfocal distance (but the full-frame format need a lens with 1.6x the focal length to give the same view).
2. Using the same lens on a small-sensor camera and a full-frame camera, the small-sensor image has 1.6x LESS depth of field than the full-frame image would have (but they would be different images since the field of view would be different)
3. If you use the same lens on a small-sensor camera and a full-frame camera and crop the full-frame image to give the same view as the digital image, the depth of field is IDENTICAL
4. If you use the same lens on a small-sensor camera and a full-frame camera, then shoot from different distances so that the view is the same, the small-sensor image will have 1.6x MORE DOF then the film image.
5. Close to the hyperfocal distance, the small-sensor camera has a much more than 1.6x the DOF of a full-frame camera. The hyperfocal distance of the small-sensor camera is 1.6x less than that of a full-frame camera.

 

[amolitor]

The whole thing is made immensely complex by the fact that nobody ever does a good job of explaining what they're holding fixed and what they're changing.

Some examples:

- in order to frame the same subject using the same lens, you have to step back when using the crop sensor, do the distance to subject is different
- in order to frame the same subject from the same shooting position, you have to use an "equivalent length" (ugh!) lens, which is shorter when you're using a crop sensor, which means (among other things) that at the same aperture (f-number) the physical size of the hole is smaller

 

[timor]

The whole thing is made immensely complex by the fact that nobody ever does a good job of explaining what they're holding fixed and what they're changing.

Some examples:

- in order to frame the same subject using the same lens, you have to step back when using the crop sensor, do the distance to subject is different
- in order to frame the same subject from the same shooting position, you have to use an "equivalent length" (ugh!) lens, which is shorter when you're using a crop sensor, which means (among other things) that at the same aperture (f-number) the physical size of the hole is smaller

OK, so CoC of one lens doesn't change on smaller sensor, so they are giving the same DoF. Unless we want the same angle of view, and for that we use shorter lens on smaller sensor, which automatically makes the aperture smaller, which gives smaller CoC, which makes the DoF deeper. Question: if we print the same size picture taken with FF and 50mm at let say f8 and picture taken with cropped sensor with "equivalent lens" and same aperture, what's gonna happen to DoF in this prints ?

 

[astroNikon]

Please, give me the link. I don't understand how the distance from the subject changes, when using crop sensor.

http://www.thephotoforum.com/forum/photography-beginners-forum/212708-so-you-wanna-see-difference-between-full-frame-cropped-sensors.html

 

[timor]

Another question: is a flange distance important for DoF ?

 

[vintagesnaps]

To me it's similar to being out taking pictures and going from using a 135mm to a 50mm; obviously the field of vision is different and what I'm recording is different. When I've been out using my digital camera which has a crop factor and one of my film cameras, I could use the same length lens and I'm not going to get the same field of view, I'm seeing something different and would have to change my vantage point if I want to get a comparable photo with both.

I've never compared flange difference in photos using different cameras that I have, probably because I haven't been out using two different SLRs at the same time; I usually take along maybe a rangefinder w/a 45mm lens and an SLR w/a 135mm, or my digital camera and a film rangefinder using the same lens where the crop factor makes a difference anyway.

 

[Ysarex84 78]

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 really...
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.

No.

I did give a very brief answer and I see I was away far too long.

Assumption: You want to take the exact same photo with two different cameras that have different size sensors. In order to take the exact same photo you will have to be at the exact same camera/subject distance. For the framing to be exactly the same you'll need to use lens focal lengths that produce the same angle of view/over sensor size. Let's pick two cameras: A Canon 5D and my little Samsung portable. In order to achieve the exact same cropping and perspective between the two cameras they must be used from the same camera position -- let's do 10 feet. To get the same framing as a 50mm lens on the 5D I'll need a 10.8mm lens on my Samsung. NOTE: In terms of content this is the same photo. The framing is the same and the perspective is the same. Andrew's right -- you got to explain what's being held constant. For this comparison it only makes sense that you're using the different cameras to take the same photo content/perspective.

I want to use the same ISO with each camera and so I will get the same shutter speed f/stop combinations for both to get a good exposure. Let's assume the f/stop then is f/8.

The 5D will give me a total DOF of 6.28 feet with the near limit at 7.8 feet and the far limit at 14 feet. My Samsung's DOF will reach to infinity with the near limit at 4 feet. There is nothing you could do with my Samsung camera to force it to give you less DOF while taking the same photo except to open up the lens aperture. If you open the lens aperture on the 5D the same amount you will be forced to always have less DOF from the 5D. You CAN NOT make the two cameras produce the same DOF taking the same photo unless you shot the 5D at f/11 and my Samsung at f/2.

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.

Joe

 

[Ysarex84 78]

Another question: is a flange distance important for DoF ?

Nope.

 

[Ysarex84 78]

The whole thing is made immensely complex by the fact that nobody ever does a good job of explaining what they're holding fixed and what they're changing.

Some examples:

- in order to frame the same subject using the same lens, you have to step back when using the crop sensor, do the distance to subject is different
- in order to frame the same subject from the same shooting position, you have to use an "equivalent length" (ugh!) lens, which is shorter when you're using a crop sensor, which means (among other things) that at the same aperture (f-number) the physical size of the hole is smaller

OK, so CoC of one lens doesn't change on smaller sensor, so they are giving the same DoF. Unless we want the same angle of view, and for that we use shorter lens on smaller sensor, which automatically makes the aperture smaller, which gives smaller CoC, which makes the DoF deeper. Question: if we print the same size picture taken with FF and 50mm at let say f8 and picture taken with cropped sensor with "equivalent lens" and same aperture, what's gonna happen to DoF in this prints ?

We don't assign CoC values to lenses. CoC values used to calculate DOF are assigned based on the size of the recording media: film/sensor. So for a 35mm film or digital FF camera a CoC value around .025mm is common. My little Samsung portable that I noted above with a 1/1.7" sensor would have a CoC value around .007mm. Going the other way a 4x5 sheet film camera would get a CoC value of .1mm and that's one of those "I can't quite put my finger on it" differences that's very real between photos taken with modern digital cameras versus photos in the middle of the last century taken with larger cameras. The history of photography is the history of shrinking media and the progressive increase in DOF in everybody's photos.

And great question about what happens when you make a print. To really deal with DOF you need to bring the enlargement size to print into the equation. If you're standardizing on say an 8x10 inch print then the smaller media has to be enlarged more and as a result tighter constraints need to be placed on the CoC value. That's why we assign CoC values to media size -- it's part of the equation.

Joe

 

[timor]

Aha. Thanks. I always though, that CoC is an optical function of the lens and actually describe the ability of the lens to create the largest blur spot indistinguishable from a point. What you said means, that a LF lens, when directed on small, 1/1.7'' will produce 0.07mm spot, when directed on FF sensor will produced 0.25mm spot, when directed on 4x5 negative will become lazy and the spot will be of 0.1 mm. So, what will be the spot on other materials like ground glass or photographic paper ? And why optical properties of the lens should change with the recording media ?
I don't think, that we assign CoC values to the media. We assign the maximum acceptable value of CoC to the media, which in turn is the minimum quality requirement for the lens for that medium. Your tiny sensor wouldn't yield any usable image, if shot with LF lens.