david_hedley Posted April 30, 2003 Share Posted April 30, 2003 My problem is getting really accurate exposure readings on macro work (around 1:1 with 4x5). I don't have the facility to take a reading direct from the groundglass, and my Pentax spot meter doesn't seem to take really accurate readings close up (it's fine the rest of the time). I have read that one solution is to use a close up lense directly attached to the meter - does this really work, and what is the best dioptre to choose? Link to comment Share on other sites More sharing options...
ken_lee11 Posted April 30, 2003 Share Posted April 30, 2003 There's no reason why the spot meter should fail at close distances. Luminance is luminance. Perhaps your exposures are wrong because you are overlooking the compensation required for long bellows extension. On the other hand, why not taken an incident meter reading ? Link to comment Share on other sites More sharing options...
david_hedley Posted April 30, 2003 Author Share Posted April 30, 2003 Ken ; I think I'm calculating both bellows and reciprocity factors correctly - the problem is that the meter doesn't adequately distinguish between areas of different luminance at close distances. Regards Link to comment Share on other sites More sharing options...
ken_lee11 Posted April 30, 2003 Share Posted April 30, 2003 This reminds me about the old story of when the thermos bottle was first introduced to a "primitive" tribesman. The explorer explained that a thermos bottle can keep hot things hot - or - cold things cold. The tribesman responded: "I have only one question: How do it know, mann... how do it know ?" In the same way, I ask about the light meter: How does it know that something is close or far away...."How do it know" ? Link to comment Share on other sites More sharing options...
bob. Posted April 30, 2003 Share Posted April 30, 2003 David, are you saying that the spotmeter's coverage area is too large to distinguish between adjacent differences in tone because the image through the meter's viewfinder is blurred and out of focus, merging the two tones together? If so, although a closeup lens will work to focus the image, you then have the problem of allowing for the light transmission through the closeup lens, but I guess that is easy to do by pointing at a grey card with & without the lens & noting the exposure difference. You will also now have the problem that your meter is very close to the subject, probably blocking the light - when I tried something similar with a 50mm lens in front of the meter, I ended up with the lens about 1cm from the subject... Cheers, Link to comment Share on other sites More sharing options...
michael_veit Posted April 30, 2003 Share Posted April 30, 2003 Is it possible that the meter is misalligned and not truly reading off the highlighted circle in the view finder? This may just be an issue that becomes more apparent at close, critically aimed distances. I'd also take great care to shade the meter from incidental light flair coming from the strong lighting frequently required for macro work. Link to comment Share on other sites More sharing options...
neil_poulsen1 Posted April 30, 2003 Share Posted April 30, 2003 I have the same meter, and I know exactly what you mean. While I haven't yet given it a try, my thought has been to get a close up lens that I can screw onto the end of the meter to see if I can get better discrimination. That may throw off my measurements, so I'll take a reading of a gray card with and without the attachment and determine a correction factor. I think you know what I mean by a close-up lens. They're sort of like a filter, except that they have no color, and they optically bend the light. For example, they make them for Hasselblad systems. The trick is to find one with the same threadsize as the meter. I've noticed that my Sekonic doesn't have this problem nearly to the same extent. But, I'm keeping my Pentax so that I can have it adjusted by Calumet with the Zone VI filters, baffling, etc. I'll use it for B&W and the Sekonic for color. Link to comment Share on other sites More sharing options...
leonard_evens Posted April 30, 2003 Share Posted April 30, 2003 David, Can you be a bit more explicit about what goes wrong? I just tried both my Pentax Digital Spotmeter and an old 1/21 analog spotmeter. I got exactly the same reading a couple of inches away as I did from six or eight feet. In any event, you should be able to use the spotmeter from further back to determine the luminance values. If necessary, use a gray card to make sure you have enough of a target to aim at. Link to comment Share on other sites More sharing options...
david_hedley Posted April 30, 2003 Author Share Posted April 30, 2003 Thanks to those respondents who are trying to understand the issue. At a distance of about 12" or greater, the meter is highly accurate and distinguishes 1/3 stop variations between areas of different luminance without a problem. At closer distances, either because the meter is not focussing properly, or because the angle of view is such that the meter is effectively merging small areas of different luminance together and giving an average reading, the meter does not seem to operate effectively. (I have both the digital and the analogue versions of the meter, and both seem to have this problem). Hence I was wondering whether using a close up lense over the front of the meter would cure this, and whether other LF photographers had found that this had solved the problem, or indeed whether there was another solution. Link to comment Share on other sites More sharing options...
tedharris Posted April 30, 2003 Share Posted April 30, 2003 David, I think you might consider a different approach. I never use the spotmeter for macro/tabletop work .... always use an incident light meter. Generally I am using strobes so I use an incident meter that can function as an ambient/flash meter. There are many on the market to choose fro, I happen to use a Sekonic 718. Most importantly, I always meter and then take a polaroid to check the lighting. There are certain types of setuos that I do so frequently for clients that one metering is generally all it takes but of 'fine arts' and other new setups I will generally shoot 2-3 polaroids befor eI am satisified with the lighting. Some of the adjustments I make are dictated by readings from the meter and some are dictated by my minds eye and what I see with modeling lights v. what I want to see and my knowledge of how to get it. A light meter, in any sort of situation where your subject is only a few feet or less away, is only going to serve as one of several tools to guide your final exposure. IMHYO for those purposes an incident meter will always serve you better than a reflected meter unless you rae metering off the gg (and even then .. and I do have that capability ... I still do both). Link to comment Share on other sites More sharing options...
leonard_evens Posted April 30, 2003 Share Posted April 30, 2003 I begin to see what you mean. The lens on the 1/21 spotmeter appears to have a focal length of about 75 mm. When closeup, you can still focus by the diopter correction lens you look through, but the distance from the objective lens to the sensor won't change. That means the image will be way out of focus and some averaging is bound to take place. If intensity differences across boundaries are important for you, it is quite possible putting a close up lens in front of the meter will resolve the problem. You should do some experiments and try it. Use a gray card and see how it responds as you cross the edge with and without a magnifying lens in front. I still think the easiest thing to do is to get back further and rely on a gray card or other substitute surfaces to help you determine what the actual intensities in the subject are. P.S. I am puzzled about why so many people spell 'lens' with an extra 'e' as in 'lense'. According to my dictionary, the correct spelling is 'lens'. Is this an alternate British spelling or something else I'm not aware of? Link to comment Share on other sites More sharing options...
tim_atherton2 Posted April 30, 2003 Share Posted April 30, 2003 "P.S. I am puzzled about why so many people spell 'lens' with an extra 'e' asin 'lense'. According to my dictionary, the correct spelling is 'lens'. Isthis an alternate British spelling or something else I'm not aware of?" Actually, it's an alternate US spelling... Link to comment Share on other sites More sharing options...
john lehman, college alask Posted April 30, 2003 Share Posted April 30, 2003 By the way, taking a reading directly from the ground glass does not require special equipment (although such is available) - any reflected meter plus a dark cloth works. You need to calculate a correction factor for the ground glass (in my case I set the meter to f/1.0 and use a +0.3 correction) and then just read the time Link to comment Share on other sites More sharing options...
leonard_evens Posted April 30, 2003 Share Posted April 30, 2003 Spelling of lens, lense This is not really related to the question under discussion here, but I was surprised to find that the newest Webster New Collegiate dictionary does include 'lense' as an alternate (American) spelling of 'lens'. But neither I nor my wife were aware of it, as native spellers of American English in our late 60s. It also isn't in my 1961 Webster New Collegiate dictionary, so I conjecture that it has developed somehow in the last 40 years. My wife who is a professional linguist of sorts suggests that it developed from the plural 'lenses' by dropping the 's'. I believe that 'lens' is still the preferred spelling in American as well as British English. Very interesting! Link to comment Share on other sites More sharing options...
Jochen_S Posted April 30, 2003 Share Posted April 30, 2003 I only have some cheaper Soligor meter, but I use it regulary with a +1 close up lens to be able to go to short distance. If the pentax has a screw in mount there will surely be some lens made for videocamcorders fitting. Suggestion: Have a quite dark room put a minimaglite without reflector in front of something black and check your meter alingment. Good luck! Link to comment Share on other sites More sharing options...
bob. Posted April 30, 2003 Share Posted April 30, 2003 "Actually, it's an alternate US spelling..." but, only if you read Websters' dictionary.... ;-) Link to comment Share on other sites More sharing options...
ken_lee11 Posted April 30, 2003 Share Posted April 30, 2003 David - I'm sorry if my response was inconsiderate or rude. I now understand the issue. Link to comment Share on other sites More sharing options...
andrew pell Posted April 30, 2003 Share Posted April 30, 2003 I might be a bit late into this thread but... Could it be that without a close-up lens, the PDSM can't focus that close (I seem to remember something from the manual about 1m+ (?) minimum focus distance as it's generally used in "infinity" situations) and therefore the image on the sensor is blurred, i.e. low contrast and it can't differentiate between tones very well so gives "mushy" readings? If you knew the minimum focusing distance, you could calculate the dioptre strength of a close-up lens to bring it down to your working distance... Andrew Link to comment Share on other sites More sharing options...
øyvinddahle Posted April 30, 2003 Share Posted April 30, 2003 Take your groundglass off, and use a 35mm camera that can measure without a lens stopped down. Canon FD, old Nikon or a pro D1. This will not work with a D100. The Canon F1N with a spot-GG can even use spot. This will take in the effect of bellows and filters. I have to refocus to get in focus, and angle the camera a bit up and down to get the highest reading. If you can not fit a lensless camera in between, use a lens and push the camera with a lens against the groundglass. I had to adjust 1.5 apertures to get that kind of reading right. If you do not have the room for a camera Link to comment Share on other sites More sharing options...
david_hedley Posted May 1, 2003 Author Share Posted May 1, 2003 Thanks again for all the input. Perhaps the attached image can explain why I want to be able to meter difference in luminance between small areas - the objective being to apply N+ or N- to the negative. The meter does not adequately distinguish the difference in luminance between different areas of the shikate mushrooms ; all I want is a method to be able to measue the difference. Link to comment Share on other sites More sharing options...
tedharris Posted May 1, 2003 Share Posted May 1, 2003 Without resorting to small fiber optic probes I am not srue any meter is going to make the differentations you seek. IMHO you will stillb e better off taking one or more incident light readings and then do your further adjusting after looking at a polaroid. The below picture was taken using a two strobe setup with a total output of 1000W and metered with an incident meter. Link to comment Share on other sites More sharing options...
ken_lee11 Posted May 1, 2003 Share Posted May 1, 2003 Apparently, the root of the problem is the lighting: how to determine the right amount of fill, because the contrast appears way too high in these JPGs. It's one thing to fiddle with N+/- development when you're in the field, and have little control over lighting (unless you're shooting a movie).... but if you have control over your lighting in the studio, then there ought to be a way to measure the values of shadows and highlights, and adjust the fill accordingly. Why stress and strain with darkroom techniques to correct the problem, when you can nip the problem in the bud ? If the meter can't get in there close enough, then try some large cards of well-known relfectivity, and meter off of them, from a comfortable distance.<div></div> Link to comment Share on other sites More sharing options...
david_hedley Posted May 2, 2003 Author Share Posted May 2, 2003 Good point, Ken, thanks. Link to comment Share on other sites More sharing options...
david g. kelly Posted May 5, 2003 Share Posted May 5, 2003 A friend of mine had the same problem. He was photographing Pez containers for sale on EBay, but he could not get accurate readings from the Pentax meter at close distances. He solved the problem by using a diopter. The Pentax spot meter has filter threads for just this purpose. Just purchase a standard diopter (+2 or +4 should do the trick) and screw it in. Link to comment Share on other sites More sharing options...
david_hedley Posted May 11, 2003 Author Share Posted May 11, 2003 Well, I just managed to get hold of a 3 dioptre 40.5mm close up filter and this more or less solves the problem - it really does work in enabling the meter to focus closer. Link to comment Share on other sites More sharing options...
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