Digital Cameras, a lie of omission.

[Bobby Ironsights]

The following post was in error. I was mistaken about one thing* Apparently, the 85mm effective focal length given by salesman and oft-quoted, is not true. When using a digi cam with a less than full sized sensor, you are not changing the effective focal length at the time of exposure, you are just magnifying a smaller picture to a larger extent when final printing occurs. I'll leave the posts up, so anyone who wants to read it can.

You know, I've seen a mistake repeated here, many times. It's the salesman that are to blame, I think.

A lie of omission.

I use film camera's, exclusively, except for my avatar, for which I used my webcam.

You see, I often hear people cleverly say, "since I have a sub-frame digi-cam, that 50mm f/1.4 prime will be like an 85mm! And that nice USM 4.5-5.6 85-200 kit zoom, now is like a 150mm to 340mm zoom! The salesman told me so! Isn't that great???!!!"

Now there is a flip side to that though, what he didn't tell you, is that the nice and fast f/1.4, will now become a turtle slow f2.4.

That kit lens? You better glue it to a tripod, because it's now a glacially slow f/11, give or take, and unlike stopping a prime lens down to f/11, that's WIDE OPEN, where optical performance is worst.

There's no such thing as a free ride. A 35mm maximum diameter exit pupil, has a limit to how much light it can let in, and a smaller sensor just wastes light.

And if that wasn't bad enough, where does all that extra light that doesn't hit the sensor go anyway?

It just bounces around, corrupting your image sharpness and contrast.

So, if you've just noticed your bum hurts, there's a reason for that....and it's not because you sat on a tack.:greenpbl:

 

[Josh66]

And if that wasn't bad enough, where does all that extra light that doesn't hit the sensor go anyway?

It just bounces around, corrupting your image sharpness and contrast.

Wouldn't the same thing happen with film? I mean, the lens is round - so there will always be extra light that isn't hitting the film/sensor.

Isn't the inside of the camera coated with non-reflective materials to prevent this?

 

[Bobby Ironsights]

Wouldn't the same thing happen with film? I mean, the lens is round - so there will always be extra light that isn't hitting the film/sensor.

Yes, of course, just much less. The lens is optimized to give an image circle just big enough to cover the film without vignetting, and not much more.

Isn't the inside of the camera coated with non-reflective materials to prevent this?

Not non-reflective, LESS reflective. There is no substance I know of which is non reflective, even black holes leak x-rays.

 

[Steph]

Now there is a flip side to that though, what he didn't tell you, is that the nice and fast f/1.4, will now become a turtle slow f2.4.

As I said in another thread, that is wrong. The max aperture only depends on the lens, not the camera you put behind it.

 

[Josh66]

Yes, of course, just much less. The lens is optimized to give an image circle just big enough to cover the film without vignetting, and not much more.

OK, I see. This -> ([]) vs. this -> ( [] ), right (hope that makes sense)?

 

[Bobby Ironsights]

absolutely, jeep grille lookalike handle dude.

 

[Bobby Ironsights]

Wrong. The focal length of the lens does not change! It is still 50mm. The field of view on a cropped sensor of a 50mm lens is equivalent to the field of view of a 85mm on a full frame, that's all. The focal length of the lens is still 50mm, whatever camera you use behind it.

Absolutely, the NOMINAL focal length of the lens when focused to infinity is technically the same, but the EFFECTIVE focal length of the lens is increased by the change in angle of view.

I can quote the basic physics here from Wikipedia.

If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene.

However, while in the case of a 35mm slr lens, the aperture area remains the same.....the ANGLE OF VIEW decreases, which, while not exactly the same as changing the focal length, has the same effect on the equation as if the focal length had changed. What I mean to say, is the effective focal length has changed.

So yes, if what you are saying is that I'm not quoting the EXACT pedantic def'n of the According to Hoyle of optics, I'll agree. But most people don't have university level physics.

Soooo... giving the simple explanation helps most people. (at least I thought it would)

The lens was designed, so the vertex of focus at infinity impacts on the film plane. In order to keep the same WORKING aperture, the smaller sensor would have to be significantly closer to the lens than the film, AND the lens would have to be designed to focus well PAST infinity to compensate for the closeness of the sensor to the rear element of the lens.

 

[Battou]

Sorry, I am a dumfck when it comes down to digital photography, but the omission of information is common place in any marketing campaign. People only want to sell stuff, not teach them how to use it or inform them there is better. They would rather wait until the sale is final and then sell you the better things. Two sales for the price of one.

People should go in knowing they are not gonna get all the information, and be prepaired to ask for it.

 

[Steph]

Absolutely, the NOMINAL focal length of the lens when focused to infinity is technically the same, but the EFFECTIVE focal length of the lens is increased by the change in angle of view.

I can quote the basic physics here from Wikipedia.

If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene.

However, while in the case of a 35mm slr lens, the aperture area remains the same.....the ANGLE OF VIEW decreases, which, while not exactly the same, has the same effect on the equation as if the focal length had changed for all practical purposes.

Of course if you use a 50mm f/1.4 lens (for example), the full frame sensor will gather more light than a cropped sensor but only because it is bigger. This has nothing to do with the effective maximum aperture changing from full frame to APS-c sized sensors. The amount of light gathered per square millimetre is the same on both sensors.

 

[Bobby Ironsights]

Of course if you use a 50mm f/1.4 lens (for example), the full frame sensor will gather more light than a cropped sensor but only because it is bigger. This has nothing to do with the effective maximum aperture changing from full frame to APS-c sized sensors. The amount of light gathered per square millimetre is the same on both sensors.

The intensity of light per square millimeter would be the same, if the effective focal length was the same. But it's not right????

Well, I've spoken to someone else about this, and there may be a fly in my ointment.

Apparently, the sensors DON'T give an effective focal length of 85mm. They just give a smaller window, on the same 50 mm focal length of 50mm.

They just crop. So the end user, when they make the final print, is having the effect of enlarging a smaller negative to fit the size of the print. That's where the only Enlarging occurs. Extra light IS NEEDED, but it comes from the "Virtual" light source, of a printer.

If you were printing film, you'd have to raise the lamphouse up higher, and leave the timer on longer.

The 85mm thing, is just a Big Fracking Lie! GRRR!

 

[Steph]

Apparently, the sensors DON'T give an effective focal length of 85mm. They just give a smaller window, on the same 50 mm focal length of 50mm.

On that we agree.

They just crop. So the end user, when they make the final print, is having the effect of enlarging a smaller negative to fit the size of the print. That's where the only Enlarging occurs. Extra light IS NEEDED, but it comes from the "Virtual" light source, of a printer.

If you were printing film, you'd have to raise the lamphouse up higher, and leave the timer on longer.

I don't quite understand this and how it relates to the maximum aperture of a lens...:scratch:

The 85mm thing, is just a Big Fracking Lie! GRRR!

Not in terms of field of view. A 50mm on a APS-c camera has the same field of view as a 85mm on a full frame camera.

 

[patrickt]

Wow, a salesman told you something that wasn't correct. Amazing. And, it must have been a lie.

A 50mm lens made for a 35mm film camera has a focal length of 50mm, a FOV equal to a 75mm, and the aperture is the same.

 

[monkeykoder]

I'm assuming here that the FOV changes but not the perspective?

 

[Sideburns]

The lens is the same, the aperture is the same...nothing changes. You just aren't getting the whole scene on the sensor, so you are getting a "cropped" view of the scene. Same speed. The focal length is not increasing, therefore the aperture is not changing. You're just cutting out a part on the edge...that's it.

It doesn't need extra light. It's the middle of the same scene that would be on film!

 

[Iron Flatline]

Halleluja. Y'all have finally figured it out. I've actually stopped participating in these threads, because no one ever seems to get it.

It's the same lens, it's just a cropped sensor. Your DOF stays the same, because it's the same data you're capturing... you're just cropping some of it. The sensor is not big enough to capture the whole circle of light coming in... it's smaller than the typical 24x36mm film negative. Which wass known as small-format until a few years ago btw. Not Full Frame.

It's not my image, but it was posted on the internet in an effort to educate people. The source is here.