Simple way to reveal image circle, possible/imposible?greenspun.com : LUSENET : Large format photography : One Thread
Not sure if this should be a practical question or not. From time to time, we may come across some LF lenses (in particular those olders) that we are not aware of their specification. With the lens in your hand, is there a practical way (this means: no need complicated methodology) to simply tell the IC approximately. Thanks!
-- Dongyun Hao (firstname.lastname@example.org), December 04, 2000
Very, very roughly, you can tell the approximate coverage by looking straight through the back of the lens, with your eye a focal length away from it, and then rotating it sideways until the aperture starts to become obscured. You can then guess, or measure, the angle that you've rotated the lens, and double it to get the angle of coverage. Doing this with the aperture wide open, and then at, say, f/22, will give you an idea of how much the coverage increases on stopping down.
Of course, this tells you absolutely nothing about the image quality, just the angle which vignetting is likely to start at.
-- Pete Andrews (email@example.com), December 04, 2000.
Dongyun: Many of this Forum's users have vast experience with older lenses. To them I suggest you put the question on any particular lens you have in mind. The following will not help you with older lenses or others for which their manufacturers do not provide performance specifications as for example Nikon and Fuji. In other cases where performance specs are available, what follows is I hope the most practical and surest way to know about a lens' real IC as opposed to the specified IC. Because the image deteriorates away from center, the point at which the lens' MTF response is zero, (i.e. the IC as given by some manufacturers) is only part of the story. More important is the quality of the image near the corners of the film. The IC spec does not tell you that. Where the corners of the film will be within the IC will depend on the size of the IC and the movements used. Should you want an IC that is, say 50% larger than the film's diagonal to allow for movements, i.e. 225mm for 4X5, we need to be concerned about the quality we can expect near the 225mm mark. With some lenses, image quality can deteriorate so much even before the lens' response is zero, that the full IC (measured according to the zero rule)is not really an equally good or even useable IC. Of two lenses for which their manufacturer gives the same IC, but which at the ICs outer limit have different MTF response, which lens has the larger practical IC? the one with the higher MTF at the outer limit. Interestingly, some of Rodenstock's wide angle lenses are specified with an IC for which the lens' MTF response at the outer limit of the IC is more than zero; Yet, other lenses for which the IC is given as marginally greater than the Rodenstock show MTFs which are zero at the ICs outer limit. The IC rating in the first case is conservative, in the second, optimistic. In reality, the IC for both is the same, even though the buyer invariably may go for the one rated as having the larger IC. The above considerations also applies to the evaluation of lenses which have large ICs but whose image quality deteriorates rapidly away from center, as is the case with Tessars, (which are great at center)the G-Claron and some wideangle lenses. One big advance in the develoment of the plastmats as for example Sironars and Symmars, is their more consistenly and excellent image quality throughout the given IC.
-- Julio Fernandez (firstname.lastname@example.org), December 04, 2000.
Thanks very much, Peter and indeed Julio, for your instructive responses! It is true that there are many kind knowledgables in this forum who have offered excellent instruction to those like me. But, the point is that not every newbi can name so many LF lenses. Also, it is difficult to expect what specific lens I can meet in lens hunting. My main purpose is to looking for a wide angle lens (I have 105mm already) for a 6X12 coverage (smaller than 4X5). I mainly shoot landscape with it and do not need much movement (personally). But, it is better suitable for using chrome film and do not want to bother using the central-ND filter due to its high cost (it is still not better than without, IMHO). Do not know if upto $500 budget (for new or used) sounds silly or not. I appreciate any votation.
BTW, I agree indeed with Julio's 'Zero rule'. But, what is the limit of a 'practical IC'? this may vary from user to user due to their variant purposes. We often see someone say that the lens is fine without the CF, but this does not necessarily hold truth for others. Even if a MTF exists, it might not give much more confidence to potential buyers without a 1st.hand using experience. This might be why there are little (if not none) Pro's who choose those 3rd party lenses in 35mm, such as Tokina, Sigma and Tamron etc., though they all have not too bad MTFs these days. IMHO, Nikon and Fuji are 'clever'. They do not give any MTF for their reputable lenses from 35mm through LF since history but let users to evaluate their own 'practical IC/quality from center through edge. We all can see that this has worked succesfully among our users. In fact, I am using true 4X5 coverage lenses for my technical 6X9 format. This sounds wasting of 'resources', but ensures the maximal optical quality at the center area of any lens made. Please critisize if I am wrong!
-- Dongyun Hao (email@example.com), December 05, 2000.
Aha! I now have a better idea of what you want. Not a rough-and-ready check for a cheap unknown lens at all.
In fact, your 6x12cm format isn't much smaller than 5x4". 5x4 has a useable area of about 95mm by 120mm, a difference of only 15mm on the diagonal. You should be able to easily cover the format with a 68 or 75mm lens, no trouble, without resorting to a centre filter, but whether you'll be able to get something of good quality, like a Super Angulon for your $500, I don't know.
I wouldn't worry too much about MTF figures. Those manufacturers that do issue them only give the theoretical design figures. In reality, decentration faults and lack of film flatness can make these graphs completely meaningless. The only real way to find out if a lens is good enough is to take some pictures with it.
-- Pete Andrews (firstname.lastname@example.org), December 05, 2000.
BTW Julio. The G-Claron is a symmetrical 'plasmat' type lens as well. It's almost identical in construction to the Symmar and Sironar, just corrected for a nearer focus, that's all. Rodenstock's Apo-Gerogon, and Konica's Hexanon GRII wide-angle process lenses have a similar specification, of up to 74 degrees coverage angle, with excellent outer field performance, and low geometrical distortion.
-- Pete Andrews (email@example.com), December 05, 2000.