[haziz]

Edit: TL; DR see Adam's post above

The amount of light entering the mirror box and falling on the sensor doesn't change (f/1.7 = f/1.7 = f/1.7). You're just using a smaller part of that total covered area. Imagine shooting the lens on film. Does the centre of the film have a different exposure? (disregarding vignetting of course)

+1000. f1.7 is f1.7 is f1.7. 50 mm is 50 mm is 50 mm.

The sensor just does not occupy as big an area of the coverage/image circle of the lens.

To the OP: I am not kidding; play around with a large format camera if you have a chance. Stick a 300 mm lens (a 300 mm lens with a big enough image circle, of course, your standard 300 mm large format lens) on an 8x10 camera and play around with 8x10, 5x7 and 4x5 backs. Or play around with a 150 mm lens on a 4x5 camera and 4x5 film plus a roll film adapter back, which may be easier to find for most people. The exposure and DOF will not change though the field of view will change with the format, e.g. the 300 mm lens is a "normal" lens on 8x10 but is a "long" lens on 4x5. The concepts become very easy to understand. And large format can be great fun!

Thanks.

Sincerely,

Hany.

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Last edited by haziz; 11-21-2013 at 10:47 AM.

[stevebrot]

To the OP: I am not kidding; play around with a large format camera if you have a chance.

Yep, working with large format or even medium format is a real eye-opener. Suddenly so many things make sense.


Steve

[m42man]

I'd just like to point out that this "discussion" arose, just because of one simple statement in post #6:

"You get the same DOF with the same lens (think of it as just cropping the FF image)."

This is incorrect: whether by mounting a lens on a crop sensor, or by cropping the image, you end up with the same result: reduced DOF. End of.

[m42man]

Read carefully at the first point of Adam's quote:

"1. For an equivalent field of view,"

That mean you use significantly wider lens on small sensor to achieve the same FOV on the large sensor (which uses longer lens), therefore smaller sensor display more DOF.

I don't need to read it carefully, I'm fully aware of that fact, and it's not the point that he made to which I was responding.

[m42man]

It occurs to me that this subject will be the cause of recurring arguments. I think the problem is that people get confused when considering the following 2 cases: same lens on a crop camera and same FOV on a crop camera. Well, I'll put it in black and white:

1. Same lens on a crop camera: reduced DOF.

2. Same FOV on a crop camera (i.e. shorter FL lens): increased FOV.

There you are, simple as that.

[Lowell Goudge]

The whole issue is that people are searching for a direct FF to APS C shooting situation. What they want is same DOF, same perspective, and same shooting distance when changing formats.

The truth is that this demand is not possible.

Perspective is simply a point of subject to camera distance changing lenses does not change perspective, changing position does.

Magnification / fov is a combination of focal length and distance, and frame size so changing lenses or formats forces a change in distance but this changes perspective

DOF is a combination of focal length, aperture, distance and total magnification from subject to camera to print. Changing any of the above changes this