Ef-s focal length

Started Jan 15, 2013 | Discussions thread
brightcolours Forum Pro • Posts: 14,436
Don't correct someone when you don't understand something yourself.

ross attix wrote:

- now multiply the 60 macros F2.8 by 1.6 to get F4.5 - DOF. So crop is deeper dof than FF 100L F2.8. So F2.8 is not F2.8 for equivalency purposes. F2.8 FF is F4.5 Crop

- now to keep the shutter speeds equal, you must raise the iso on the FF camera. You must raise it by 1.6 squared. So 1.6x1.6= 2.56. So if your crop camera is ISO = 100, you must raise the FF camera to ISO 250.

You should have quit after the crop explanation because the rest of this is simply incorrect, and it confuses folks who are trying to figure it out.

He should not have stopped, as what he wrote is correct.

There is no difference in f/stop or shutter speed or DOF in this comparison.

1- The exposure is determined by the light falling on the scene and how the photographer wants to represent it. If the exposure you like at ISO 200 is 1/125, f/8, that exposure will be the same across any camera/lens combo.

You are wrong here already. How the photographer wants it? That would then encompass FOV and DOF. The only two tools the photographer has that determine how the image is rendered.

So, different f-values are needed, for the same DOF. They are called equivalent f-values. And different focal lengths are needed, for the same FOV. Those are called equivalent focal lengths.

Exposure is determined by a combination of exposure time and amplification of the signal (changed by ISO setting).

So, for equivalency purposes, a 60mm lens is used on APS-C and a 100mm lens on FF. Equivalent focal lengths for those two different formats. Both will give a similar FOV.

For a similar DOF, use equivalent f-values. if you want to use f8 on APS-C, the equivalent f-value on FF would be f8 x 1.6 = 12.8, so f13.

If you feel the need to have the same exposure time, set the ISO settings to equivalent settings. is you use ISO 200 on APS-C, with 1/125th sec exposure time, set the equivalent ISO setting on FF of 200 x 1.6 x 1.6 = ISO 512 or the closest to that.

It is misleading to tell somebody that the f/stop changes and they have to adjust the shutter speed or ISO to correct for it. To follow your logic, your exposure would change when you change lenses. It doesn't. The light is the light.

You do not get at all what is talked about. We are talking about equivalent lenses on different sized sensor-bodies. We are not at all talking about changing lenses on the same body.

2- DOF is a function of the lens focal length; the distance to the subject; and the f/stop being used. A crop sensor just has less image size. There is no change in DOF at the same lens to subject distance switching between a crop and FF body.

DOF is a function of aperture size (nothing else, if we take a print size and view distance that will not change from image to image).

There IS a change in DOF when using the same lens on different formats. Same lens on APS-C will give a more shallow DOF than on FF. Also it will give a much more narrow FOV.

3- All of this about changing ISO is wrong as well for the same reasons.

It is not wrong, just because you do not understand what is being discussed. We are discussing equivalent lenses for different formats. And as side issue, equivalent ISO settings for different formats. What the poster you replied to posted was correct and not strange or wrong.

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Ross Attix

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