ISO noise versus MTF drop at wide apertures

[todd_west]

This is something of an idle curiosity, but since I'm still waiting

for my DSLR to show up I'm wondering what people's experiences have

been with trading off aperture for ISO for low-ish light

photography. Given something around EV6 at ISO 400 and good glass,

like an 85 f/1.4 and a 70-200 f/2.8 zoom, which would result in

greater percieved sharpness when using a current generation DSLR:

 

1) shooting 1/15s at f/2 and ISO 400 on the prime or, 2) shooting

1/15s at f/4 and ISO 1600 on the zoom

 

The zoom has greater DOF and is at a smaller aperture, so it will

almost certainly achieve a higher MTF than the prime. The question

is wether the resulting MTF increase is large enough to offset the

increase in noise from the higher ISO. The presence or absence of

subject motion and putting the camera on a tripod won't matter much,

since the shutter speed is the same with both lenses.

 

After looking through the ISO image sets at dpreview, my personal

guess is the prime is likely to come out slightly ahead. Your

thoughts?

[jackflesher]

Just like with film, you'll be better off with the lower ISO and faster prime lens. However, if the zoom is a Canon IS lens, then you really do gain an easy two stops of hand-holdability...

[jbq]

Until you apply some post-processing, the "theoretical" MTF of the sensor is going to be the same. After you apply some kind of noise-reduction the MTF is going to drop a bit. "perceived" MTF isn't as high as theoretical MTF before you apply noise reduction, but it gets affected less by the noise reduction.

 

Depth-of-field is definitely a major concern. You don't specify which camera you're getting, so I'll assume it's a D70 (pretty much the one with the largest pixels out there). The D70 has significant response at 48 lp/mm (as a rule of thumb I expect that a color DSLR needs 2.66 pixels to "significantly" resolve a line pair). At 48 lp/mm, a circle of confusion of 0.15mm already eats half of the transmittance (and 0.15mm is only half of the "standard" 35mm value) - if you want to use 35mm DoF scales *and* really maintain a top MTF on a D70 you need to stop down 2 1/2 stops more than the table says. You're not saying what the subject is (i.e. the focusing distance), but with an 85mm at f/2 and a 0.12mm CoC the hyperfocal distance is about 1000 ft and the depth-of-field at 7ft is about 1.2 inches.

 

In case you're worried about diffraction and do not want to stop down, don't worry, you can safely stop down to f/10 without any significant diffraction effect.

 

I'd say that the final comparison really depends on the subject.

 

BTW, I found that applying noise-reduction only in the areas with little detail worked very well, and that applying levels to drop dark noisy areas to be entirely black could also give very good results.