Why don't you just step back a bit for a crop body? compared to a FF.

Started Apr 26, 2010 | Discussions thread
Crocodile Gena Senior Member • Posts: 1,017
Re: A bit o' physics

LeoR wrote:

That's not a "bit o' physics", that's a lotta physics.

My apologies.

I always thought that by changing aperture, while keeping shutter speed (and other variables like ISO) the same, results in a change in exposure.

It does. It's just that exposure is not relevant -- total light is the relevant measure.

In your previous post, you mentioned that

..."if the FF camera is at 50mm f/1.4, the crop camera is once again SOL because it cannot get an aperture large enough to make up for the greater distance".

In other words, the crop camera needs a larger aperture to make up for the greater distance.

Yes.

I can't understand all that physics mumbo jumbo so if someone can post a real world example of this, that would be great. Just something simple like a picture of a flowerpot from 10 feet with a full frame and 16 feet with a crop frame (to give equivalent framing) using the same lens, ISO, light source position and shutter speed.

If I am interpreting the quoted statement correctly, the EXIF data should show something like 1 or 1-1/2 stop difference between the two shots. Is that right or am I still not getting it?

It would be 1 1/3 stops. I don't have an example of this to post (if ljfinger is reading, however, he might be able to post some), but I can give some "thought experiments".

Let's consider the following:

5DII: 10 ft, 50mm, f/2.8, 1/100, ISO 100
7D: 16 ft, 50mm, f/1.8, 1/100, ISO 100

The situation above represents the same framing and DOF, since the 7D has a an aperture diameter 1.6x the 5DII to make up for the 1.6x greater distance. Clearly, if the 5DII image is correctly exposed, the 7D image will be overexposed. The 7D must then shoot 50mm f/1.8, 1/250, ISO 100 to not overexpose, but now the image is made with less total light, and is thus more noisy.

Next scenario:

5DII: 10 ft, 50mm, f/2.8, 1/100, ISO 400
7D: 16 ft, 50mm, f/1.8, 1/100, ISO 160

In this situation, the framing and DOFs are once again the same, but this time both sensors absorb the same total amount of light without oversaturating. The exposure is 1 1/3 stops higher on the 7D (remember, "exposure" does not include ISO), but by adjusting the ISO accordingly, the apparent exposures are then same.

Thus, we see there are circumstances when 1.6x can match FF, and circumstances when it can't. I'll leave it as an exercize for the reader to verify that if the 5DII is at 50mm f/1.4, then the 7D is SOL no matter what.

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