Image circle

[alexo]

<p>Hi,<br /><br />I am considering getting a PB-4 bellows and I was wondering if there is a formula to calculate the size of the image circle based on the lens extension. I want to know how much I can swing and tilt the bellows without actually doing it.<br>

Thanks</p>

[peter_langfelder]

<p>I __think__ the image circle diameter D with extension L would equal (image circle without extension)*(L+ exit pupil distance)/(exit pupil distance). The image circle without extension should be about 43mm (could be more on some lenses). The problem is finding the exit pupil distance, most manufacturers do not seem to specify it in their data.</p>

[michael_bradtke]

<p>As an owner of a PB-4 it has been my experience that I run into mechanical vignetting from the bellows itself before I ran out of image circle. The other drawback to using a bellows on a DSLR is all the crap you get on the sensor from the inside of the bellows.</p>

[alexo]

<p>I currently use the Canon Autobellows (old FD mount) with adapters and I've never gotten crap on the image sensor from the bellows. <br>

Did it matter which lenses you used and what extension you had or was the image circle always big enough regardless of the focal length and extension?</p>

[michael_bradtke]

<p>I would say you have been lucky to not get crap on your sensor from the bellows. I am not sure of the final magnification that I was getting. I used my 105 f/2.5 for a lot of the stuff I did as well as a reversed 24mm f/2.8. The 24 was a hard to get a sharp image with. A car driving buy my house at the wrong time was enough to degrade sharpness.</p>

[Two23]

<p>As a long time 4x5 camera user, I have to wonder here if you are calling "image circle" the circle that is merely illuminated? With 4x5 lenses, image circle refers to the part of the image circle that is SHARP and corrected for aberrations.</p>

<p>Kent in SD</p>

[alexo]

<p>Hi Kent, <br /><br />That is what I mean.</p>

[rodeo_joe1]

<p>As a rough guide, the image circle diameter will double when an extension equal to the lens focal length is applied. The exit pupil distance can pretty much be disregarded with "normal" i.e. non-retrofocus or telephoto lenses. However, as others have said, mechanical vignetting from the bellows will most likely occur before you can use anywhere near that amount of tilt/shift.</p>

<p>FWIW, my own preference for bellows use is to use good-quality enlarging lenses (Rodagons or Componons), which nearly always have a straightforward optical design, and most come in a standard Leica thread. The image circle on these lenses is also quite wide. I've found Rodagons to give generally superior macro results. The simple construction and compact barrel of enlarging lenses doesn't put much strain on a bellows mechanism either; unlike using some of the monster SLR macro lenses that are around now. Their other main advantage is that they're dirt cheap to buy used in mint condition. Nobody wants them these days.</p>

<p>The answer to not getting muck from the inside of the bellows on the sensor is simply to clean the bellows! You can get miniature AA battery powered vacuum-cleaners that are ideal for the task, or use a small paintbrush to dust off the inside of the bellows. My bellows (and dusting brush) are kept in strong "ziplok" plastic bags when not in use.</p>

<p>Incidentally, an increase in image circle is generally a bad thing as far as image contrast is concerned. I've noticed that lenses with a large image circle used on a bellows will create a lot more non-image forming flare - no surprise - and degrade contrast. I've been meaning to experiment with a black-velvet snoot inside the bellows and behind the lens, but haven't got round to it yet.</p>

[Two23]

<p>It would make sense that an enlarging lens would have a large sharp circle. They are making large images, after all. I think this would be something to explore.</p>

<p>Kent in SD</p>

[rodeo_joe1]

<p>I'm guessing you're now eyeing up enlarging lenses for large-format use Kent? Because of the bellows extension used, you can get away with quite short focal lengths at macro distances. For example, an 80mm f/4 Rodagon at 1:1 will cover 5"x4" with a bit to spare. Of course it becomes an f/8 lens and a bit dim to focus, but the loss of two stops is common to all lenses at lifesize mag.</p>

<p>I just had a quick peek at what 80mm Rodagons and Componons are fetching on *Bay, and the price seems to have gone up to around £100 (~$150 US). That's 4 or 5 times more than what I paid for mine a few years ago. Maybe people are catching on to the usefulness of these little gems. So good hunting!</p>

<p>PS. Hoya ELs and Minolta enlarging lenses are good too, and have a lesser reputation and hence lower price - The Hoyas especially are totally under-rated. Komura made a 6 element Komuranon-S 50mm f/2.8 enlarging lens that's great too. Unfortunately I've never seen one in a longer focal length. OTOH El-Nikkors have an inflated price tag due to their reputation for quality. Undeserved IMHO in the case of the first issue 50mm lenses.</p>