Interchangeable lens camera when aperture size remains constant!

[Shangheye]

So I tgot tis Diana F+. Loving it and all my expsoures seem to be spot on for all the settings, given I was using an ISO 400 film (I believe the shutter speed is around 1/60-1/100s) and the apertures on the camera are as follows": Cloudy - f11, Partially Cloudey - f13, Sunny f-19 and a Pinhole setting of f150.

Now here is my slightly technical question (I know bizzare to say that about a Toy Camera)...

All the above f numbers are for a lens on the camera of 75mm. On the Diana F+ you can change the lens (e.g. to a 38mm lens) while the Aperture remains a part of the body. Given the focal length has changed, and the aperture diameter has stayed the same for the various settings...does that mean the f number has changed?

i.e. I now has a 38mm f5.6, f6.8, f9.6 for the same settings? (Using the f numbers claimed by the Lomography group, I have calculated the aperture diameters to be 6.8mm, 5.7mm and 4mm)

I know this may seem like overthinking it, but I have a project coming up with the camera, and I would rather not come back with massively over-exposed negs (in this case possibley up to 2 stops!!).

Rgds, Kal

 

[Christopher Walrath]

The aperture value is a ratio between focal length and aperture diameter. A 75mm lens with a body aperture value of f/11 has an aperture diameter of just under 7mm (75/11=6.81). That same aperture (part of a camera body and not the lens) with a 38mm lens would give an aperture value of about f/5.6 (5.6*7=39.2). If the aperture is part of the lens then the aperture value would remain the value published on the lens.

 

[Shangheye]

That's what I was thinking, but that is not referred to anywhere on the web...i.e. that as you change lens on the Diana so the f number changes....two stops is a massive amount Toy camera or not....thanks for confirming my view. Rgds, Kal

 

[2F/2F]

One VERY easy way to find out for sure: Take a pic with the attachment off. Then take the same pic with it on. Finish the roll any way you like and process. Compare the negatives.

 

[Shangheye]

Thanks 2F...I have switched lenses half way through the test roll and will take a couple of shots at various settings just to be sure. Rgds, Kal

 

[ic-racer]

If you have a way to check the size of the entrance pupil as viewed from the front of the lens at the various settings (like a "B" setting), you can calculate the F numbers from there.

 

[Shangheye]

That is what I have done ic-racer..as per the measurement above which is consistent with what I see with my eyes. Need to remember that when using the 38mm, ISO100 film will be better for most uses. K