7d

[dalsimer]

I own a 5d mark 2 .....Love it ,...
Heres my question if i purchase a 7d and put my glass on the 7d does my wide open aperture change ?

for example
I own a 200 1.8 if i bought this on a 7d do i now have a 320 1.8 due to the magnification ?
or do i lose a partial or full stop
I have asked may people and i seem to be getting different responses.

 

[esbenlp]

If you put your 200mm 1.8 on a 7D you do indeed get an effective 320mm with an aperture of 1.8. The aperture does not change. The thing is, that because the sensor on the 7D is cropped, the DOF is not as shallow as on your full frame 5DII. This means that if you put a 35mm 1.8 lens on the 7D (roughly equivalent to a 50mm focal length on FF) and a 50mm 1.8 on the 5DII, the 5DII would render the image with a shallower DOF. I don't know if this will be visually detectable on photos with the 200mm 1.8, as the effective focal length is 1.6x longer on the 7D. I would imagine that the gain in focal length from the 7D makes up for the less shallow DOF of the sensor, so you effectively get the same shallow DOF in photos.

I hope that this wasn't too confusing and that it might help you.

 

[mrjimmy]

I am not a expert. But the f/1.8 will remain the same. Yes it will go from 200mm to 320mm by cropping not by magnification. Remember crop frame cameras don’t magnify the image. It just looks that way. If you add a 1.4 converter of some type that magnifies the image then yes you lose a f stop. The DOF is shallower on FF but at some point in the affect of adding MM by 1.6 crop it will look the same. Or so I would think.

 

[Admin US West]

I own a 5d mark 2 .....Love it ,...
Heres my question if i purchase a 7d and put my glass on the 7d does my wide open aperture change ?

for example
I own a 200 1.8 if i bought this on a 7d do i now have a 320 1.8 due to the magnification ?
or do i lose a partial or full stop
I have asked may people and i seem to be getting different responses.

You will get difference responses primarily over definition of terms. Words like magnification set off those who want you to use their term, same with depth of field. Everyone is saying the same thing, but arguing over terminology and conditions.

Check wikipedia for information, its usually more accurate than information you receive from forums.

Aperture: Its a property of the lens, and does not change if a different camera body is used.
http://en.wikipedia.org/wiki/Aperture


Crop Factor or focal length multiplier.
http://en.wikipedia.org/wiki/Crop_factor

 

[gene_can_sing]

The 1.6 crop makes a big difference in terms of DOF.

At the same F-stop (say 2.8), the DOF is going to be considerably shallower on the 5D.

Also, the 5D has a much larger sensor so it's low light is much better. F2.8 on the 5D will be considerably brighter than the same f-stop on the 7D in low light situations.

Not sure what the exact math conversion is, but maybe the 1.6 applies to the DOF and the low light also?

 

[Macadameane]

I know this horse has been beat many times, but the size of the DOF does not change with sensor size; but with the lens and distance to the subject.

On a full frame camera, you must get closer to your subject to have the exact same framing (with the same lens) on a crop sensor, this (moving closer to the subject) is what causes the DOF to be more shallow.

Explained well with examples in this video:
http://www.youtube.com/watch?v=L0n4pyhx7Dk

 

[AJ]

No, no, no.

The focal length does not change. Focal length is a piece of physics that's part of the lens design.

Aperture does not change. Again, this is part of the lens.

Fstop does not change because the previous two items are fixed. fstop = focal length divided by aperture.

Field-of-view will change because of the crop factor.

Given the same focusing distance, depth-of-field will not change.

Given the same framing, depth-of-field will be greater on the 7D because your focusing distance will be greater (due to the crop factor, you'll have to back up)


I hope that helps.

 

[dougkerr]

Given the same focusing distance, depth-of-field will not change.

That's not necessarily true. The reckoning of depth of field is based, among other things, on an arbitrary criterion we choose, the circle of confusion diameter limit (COCDL). This is the largest diameter of the blur circle resulting from imperfect focus of objects not at the ideal focus distance that we, based on some outlook, are willing to consider "acceptable".

We may use many premises to choose a value of that. One traditional premise is as a certain fraction of the frame diagonal size. The concept behind this a fixed angular size of the "blur circle" when the image itself is viewed with a consistent angular size, for example, printed to a certain size and viewed from a certain distance.

If we follow that premise, then for a camera with a "smaller format", with a certain focal length lens, a certain f-number, and a certain distance to the subject, the calculated DoF will be smaller than for the larger-format camera - not because anything has changed in the optical physics but rather because our criterion of acceptable blur is changed for the new format size.

Given the same framing, depth-of-field will be greater on the 7D because your focusing distance will be greater (due to the crop factor, you'll have to back up)

Indeed.

But it is interesting to see how that happens. Two factors are at work.

Firstly, if we indeed hold to the choice of a COCDL that is a fixed fraction of the fame size, then, for a "smaller" frame size, that factor of itself will decrease the calculated DoF.

However, as you point out, to maintain the original framing, the needed distance to the subject will increase. This increases the calculated DoF, and by a greater ratio than it is diminished by the adoption of a smaller COCDL. Thus, the net effect is indeed an increase in the DoF.

Best regards,

Doug