View Full Version : 150mm G-claron a couple of questions
I've got a 150mm claron coming that I picked up off Ebay. I already have a #0 shutter that currently holds the cells for a 127mm Xenar. Obviously the F/stops won't match up but can I just do some math?
If my math is right:
127/labelled f/stop = aperture size.
150/aperture size = new f/stop
So an indicated F/16 would be F/19 with the 150mm lens. F/11 would be F/13. F/22 would be F/26. Sort of makes that digital light meter make sense.
The lens uses 35.5mm filters? Anybody know of a source for a setup ring to something more mainstream?
Hi Nick. Yes, 2/3 stop. When the scale says you're at f10, the 150 will be at f12. Guaranteed to make your brain hurt when you're trying to also remember a dozen other things necessary for the shot. Tiffen made a 35.5 to 46 step up but they're hard to find. Excellent little performer that. Another way to remember is; 127 lens 5" 150 lens 6" f5:f6~f10:f12
Here's a link (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2957235303&category=15215) to a step up ring. Evidently olympus is making them for a digital application.
I'm just thinking of making a little chart up with F/11,F/16 and F/22. The chart would have what they really are. The lens didn't cost me much and the shutter was free so I'm hoping I've solved my need for a 150mm with a little more coverage then the 150mm Xenar I have.
You could also just make the adjustment on film speed when metering. Example: if you are using iso 200 speed film, set the light meter at ISO 160 and use the given values.<br/><br/>I have a polaroid copal #1 with a smaller-than-notmal aperture opening that I will be using with a 210mm caltar. Thats not going to be fun to calibrate but i cant afford a real copal #1 yet.
That means that an f19 for the 150mm lens corresponds to an f16 on a shutter designed for a 127mm lens, etc. So, if your meter reading gave you f19, you would need to OPEN the aperture up to an f16. The aperture must get larger, because the 150mm lens is further away from the film than the 127mm lens. Otherwise, the film won't get enough light.
This corresponds to log(150/127)/log(1.41421) = .48 stops, or effectively a half-stop. (A stop difference on an aperture scale gives you a ratio of 1.41421 between the two aperture readings. 1.41421 is the square root of 2.) So, if you obtain an f22 on your meter as the correct aperture, then set this aperture on your shutter and OPEN by a half-stop. It will always be a half-stop correction. A chart is not really needed.
The problem here is that it isn't the actual aperture that matters, it is the effective aperture, which is the size that the opening appears to be when looked at through the lens from the front. This differs from lens to lens. There are ways of measuring this, some of which have been suggested in earlier threads, but using your light meter to make comparisons between the lens in the original barrel, and the lens as mounted in the shutter, is probably the best bet.
With the lens in barrel, focus at infinity and point the camera at a plain wall. Using your dark cloth to exclude extraneous light, take a series of readings from your groundglass at each stop. Change to the shutter, make sure that the ambient light hasn't changed, and repeat the process, stopping down until you come to the same reading obtained earlier, and marking each stop as you go.
While Ernest is right that the f-stop is determined by the diameter of the entrance pupil and not the physical aperture diameter, im many (but not all!) cases it is not necessary to measure the actual amount of light. For many lenses the lens cell mount itself defines the maximum opening of the entrance pupil: when looking through the front of the mounted lens, shutter open, you close the aperture until the blades just become visible. That is your maximum aperture position, i.e. f/9 for the G-Claron.
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