Tuesday, 15 April 2008

Bessa useful lens limits

To add a little more to flesh out this camera to a potential buyer (for using not for those who simply collect), I thought I'd explain that this camera will simply not give sharp images from edge to edge at less than f11.

There is debate about the Vaskar lens on the camera, and while some say that the lens isn't sharp other swear that it is. Here I'll say that it can be and that it depends on the photographers choices.

Here's why.

For large format camera users looking at the film plane with ground glass is a normal and even essential thing to do. Large format cameras even have the corners cut off on the ground glass, making it easier to actually do this.

The reason you'd want to look here is to see if you've moved your lens too far, and run out of the area where the image from the lens actually falls (you see, large format cameras move the lens around to give control over focus and perspective see this explanation). So I popped open the back of the Bessa early on in the piece to see what I can see. I've done it again here to explain my assertion above.

In this image, I've taped a bit of translucent (foggy) plastic sheet to allow you to see the image formed by the lens, and the lens through the corner of the frame.

You can just see the lens through the torn off corner of the palastic and you can see its not quite perfectly round. Looking at the image below, taken at f4.5, you can see that the shape made by lens is not perfectly round. This will mean that less light will fall on the edges of the film than in the middle. This will make the image darker at the edges (vignetting) and will also reduce the image quality too.

This image is taken at f8 and you can just see that the the lens is still slightly obscured in the bottom third at the left.

its not until you get to f11 that the lens circle is now nice and round

So basically this means that the lens needs to be stopped down to about f11 or lower if you want edge to edge uniform lighting, or even edge to edge sharpness. You'll notice we're not right at the corner of the film plane here, so to be on the safe side stopping down a little more is needed. In practice I've found that you need to stop down at least f16. So, essentially this means that this is a "sunny 16" kind of camera or use a stable support (Eg tripod or solid base) and then use f16 (preferably smaller) and 1 second or longer so you can get reasonably accurate exposures.

Further, there is no accurate markers in setting the aperture (no dent to feel the position of the aperture setting as in more modern cameras) and no 1/2 or /1/3 stop markers. Meaning you've got to 'guess' the adjustment and being more accurate than full f-stop is challenging (meaning that trying to set 1/2 or 1/3 stops is guess work). This is ok with black and white, and possibly colour negative, but I think it makes a challenge for slide or 'positive' film.

However, if you can set it at f16 or f22 (and use a tripod) it gives very nice results. Like this close focus example at f11:

You can see (if you click on the image) that the edges are starting to fuzz ... this was exposed at f11. Below is and a segment from the center of the negative. Please click on either to see larger versions.

So, if you can put up with the difficulties of using this camera:
  • limited useful f-stop range
  • single focal length
  • no light meter
  • focus by number (needing some way to correctly pick the distance)
  • no 'dent' to be reliably sure of f-stop selected
  • no partial f-stop marks
then it can provide you with excellent images.

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