Helios 58mm f/2 and Voigtlander 58mm f/1.4 depth of field

... of course I realise that the depth-of-field of these two lenses should be similar at a given aperture - but the apparent depth-of-field through the viewfinder is quite disimilar. The two lenses are mounted on different cameras: Helios 44-3 on Chinon M1 and the Voightlander on a Nikon FM 2 with grid screen. Informally assessed at full aperture for both lenses (f/2 vs f/1.4) the Helios has shallower depth-of-field. I assume this is mostly down the the different focusing screens of the two cameras: a coarse one in the Chinon and a very clear one in the Nikon. Maybe the focal lengths are different too or is it a genuine effect caused by the highly unusual 'swirly' Bokeh of the 44-3 ?

could the magnification of the viewfinder affect too?

Both lenses should have the same depth of field at f/2, the focal length variation wont be off by more then a couple of percent at max (thus negligible difference to dof)

What you are seeing is indeed the effects of the focus screen, some focus screens are optimized for brightness and dont actually show any smaller dof then f/2 (or on modern DSLR screens f/2.8 even).

Try setting your f/1.4 lens to f/2 and press the preview lever, its possible you wont actually see a difference.

..I've tried stopping the Voight' to f/2, as suggested, and no difference in dof or brightness can be discerned. Also tried the Helios on the Nikon (it can be held within the F-mount) where it focuses to about 5 ft when set to infinity. The observed bokeh is quite different compared to the Chinon M1. The Chinon vf is also a lot dimmer and with less coverage.

filmconquistador said:

could the magnification of the viewfinder affect too?

Click to expand...

The quality of the focussing screen may well affect the apparent DOF too. The M1 screen will almost certainly be a lower quality than the Fm2 which will affect the apparent sharpness. The best way to test would be to run a film through with both lenses at wide apperture say F2 and upwards and that way you will definitely see the difference, if any. Of course the quality of the two lenses may also have some bearing on the result.