Crop factor for aperture question

DoctorJ

DoctorJ

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I was reading something about an f1.8 Nikon lens in NPhoto magazine and it stated that on a crop sensor it gave an effective focal length of X (whatever it was, 1.5 x the focal length), which I know all about, but also went on to say that it gave an effective widest aperture of Y (I can't remember what it was) but effectively said that wide open the aperture was smaller than it would be on full frame, ie a 'crop' factor for the aperture as well as the focal length. I have never heard of this before and have never seen it mentioned on here either. Is it true?
 
very simply - yes, but its not quite that simple

on a crop the aperture needs to be multiplied by the crop factor to give a equivalent depth of field on FF, so an f/1.8 lens on a 1.5x crop would give a DoF equivalent to f/2.7 on FF (at the same focal length.

But for calculating exposure f/1.8=f/1.8 regardless of crop.
 
You need to read my depth of field thread just a little bit down as it will tell you why this is the case :D

boyfalldown is right in the maths.
 
Yes it is, for example 50mm @ f/2.8 on a D90 would have a similar look to 75mm @ f/4 on a D700. It's the same with those compacts with fast f1.8, f/2 lenses - the wide end (6mm) may be equivalent to 28mm in full frame terms, but you get the kind of depth of field you'd get at f8 or f9.

Edit - remember to refresh before submitting :bonk:
 
Saw your post on DoF and thought it was a great piece of work - thanks for taking the time and effort to share it, I will read it thoroughly
 
DoctorJ said:
Saw your post on DoF and thought it was a great piece of work - thanks for taking the time and effort to share it, I will read it thoroughly
Click to expand...
Thank you.

If it still leaves questions, please let me know, I'll try and update it with answers...
 
In case you hadn't, you also need to separate the DoF related to the aperture and the light capturing ability.

1.8 is 1.8 as far as exposure is concerned so that stays the same on crop or full frame.
 
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