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Photo Forum / Digital Photography / DSLR Cameras / December 2005ISO setting on a dslr?
Why is there an ISO setting for a dslr? I always set my ISO setting for tha appropriate film I used, but without film, why bother with ISO? Thanks in advance. >Why is there an ISO setting for a dslr? I always set my ISO setting for tha >appropriate film I used, but without film, why bother with ISO? > >Thanks in advance. Increasing the ISO will permit you to use a faster shutter speed with the same aperture or to use a smaller aperture with the same shutter speed. This is useful in situations where flash is not permitted, for example, churches or museums. Increasing the ISO may increase the noise in the photograph. Jack Thank you Jack. Is there a standard ISO setting for a dslr? >>Why is there an ISO setting for a dslr? I always set my ISO setting for >>tha [quoted text clipped - 10 lines] > > Jack >Thank you Jack. Is there a standard ISO setting for a dslr? Not really. I keep my ISO as low as possible. Jack > Thank you Jack. Is there a standard ISO setting for a dslr? In theory, 100 gives less noise than 200, but in practice it's very hard to tell the difference. On Canon 350D, 20D etc 400 is also VERY good - 800 starts to show a bit of "grain" (but it really depends on how big the photo is printed/displayed) - and 1600 / 3200 (20D) in my opinion look pretty rough, but you can have a good degree of success cleaning up in photoshop using noise filters. >> Thank you Jack. Is there a standard ISO setting for a dslr? > [quoted text clipped - 4 lines] >rough, but you can have a good degree of success cleaning up in photoshop >using noise filters. It's not just the ISO per se that determines noise; it also depends highly on the relative exposure level at that ISO. An ISO 1600 shot of a white object with grey lettering on it, exposed well, will have less noise than an ISO 200 shot of the same scene that is average-metered on a DSLR with relatively clean amplification. Looking at it another way, the absolute exposure determines the original signal-to-noise ratio in the sensor, and the higher the ISO, the more numbers are used to represent luminance in any given shadow range, and the cleaner those shadows will be. Low ISOs are never a good alternative when they result in under-exposure. Some people have accused me of looking at things backwards, but really, this is how things are really happening in the camera. An ISO-independent exposure is taken in the sensor, based soley on subject lighting, f-stop, and shutter speed. At this point, ISO only has one, single and indirect effect - it affects metering. ISO does not play a role again until the data is read out and digitized; at ISO 1600, about 1/16th the range of sensor voltages becomes the RAW numbers up to 4095 (through higher amplification); at ISO 100, roughly all of them. So, if your sensor exposure only uses half the range, shooting at ISO 100 would be image-destructive, as compared to shooting at ISO 100. If only 1/16th the range is used, ISO 1600 will give the highest quality results. Once you understand this, you can make more optimal exposure/ISO decisions.  Signature<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<> John P Sheehy <JPS@no.komm> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>>< >So, if >your sensor exposure only uses half the range, shooting at ISO 100 would >be image-destructive, as compared to shooting at ISO 100. Whoops! That last 100 should have been 200. Signature<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<> John P Sheehy <JPS@no.komm> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>>< I (think I) understand what you're saying, but ... ... let's see if I've got this right ... "If I take 6 shots - one at each available ISO on my 20D - and each are correctly exposed so that the full tonal range is used - then the lower ISO will always have the lower noise". However ... "If I'm struggling to get a shot in poor light - in terms of noise - I'm always better off to use a a higher ISO than to have an image more than a stop under exposed at 1/2 that ISO"? Cheers, Colin > ... let's see if I've got this right ... > [quoted text clipped - 7 lines] > always better off to use a a higher ISO than to have an image more than a > stop under exposed at 1/2 that ISO"? Right. To put it another way, underexposing at low ISO in an attempt to avoid "high ISO noise" is not a good idea, and will result in more noise than using a higher ISO. SignatureJeremy | jeremy@exit109.com >However ... > >"If I'm struggling to get a shot in poor light - in terms of noise - I'm >always better off to use a a higher ISO than to have an image more than a >stop under exposed at 1/2 that ISO"? Exactly. A lot of very poor exposures have resulted because people don't understand this, and try to stick to low ISOs unless it results in obviously dark images. Signature<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<> John P Sheehy <JPS@no.komm> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>>< > In message <HZnsf.10884$vH5.528938@news.xtra.co.nz>, > Exactly. A lot of very poor exposures have resulted because people > don't understand this, and try to stick to low ISOs unless it results in > obviously dark images. How much "headroom" is there between what Canon call "a standard exposure" and a blown highlight? I always use the lowest ISO that will give me the minimum acceptable shutter speed - although it's "a given" that the metering/exposure will be set for a standard exposure. >> In message <HZnsf.10884$vH5.528938@news.xtra.co.nz>, >> Exactly. A lot of very poor exposures have resulted because people >> don't understand this, and try to stick to low ISOs unless it results in >> obviously dark images. >How much "headroom" is there between what Canon call "a standard exposure" >and a blown highlight? If you use the camera's metering on a grey card, then a 90% white will be about 1 stop below clipping in the green RAW channel, 1.5 stops below clipping in the blue, and 1.9 stops below clipping in the red, with the 20D, in daylight with daylight white balance. Some of the cameras will have a little less headroom in the blue channel, like the 10D/300D, which will have only about 1.1 stops. With high-key (mostly white), or low-contrast scenes (or any scene where the brightest areas dominate the frame), using the matrix metering, it is safe to go to +2 EC or sometimes even higher (in manual mode). >I always use the lowest ISO that will give me the minimum acceptable shutter >speed - although it's "a given" that the metering/exposure will be set for a >standard exposure. The cameras still meter much like they were doing so for film; film records best in the middle zones, generally, and slide film is meant to be projected without processing, so camera paradigms and common wisdom often forget that digital is best just short of clipping, generally speaking, in both the sensor, and in the RAW data. Low ISOs are great - if you can do them without under-exposing, blurring, or losing too much DOF or sacrificing lens quality to max apertures. The photon budget is often restricted, however. I'd shoot everything at ISO 200 on my 20D if I could, but I can't. You have to learn by experience what the trade-offs are between aperture, shutter speed, and ISO, but one thing is clear; you get the highest quality recording at any given combination of shutter speed and aperture when you use the highest ISO that doesn't clip desired highlights. You can always do better than an ISO 100 with a histogram that is empty at the right, at a higher ISO. Of course, the histograms on our cameras are for JPEGs, and not for the RAW data, so they aren't always clear. I keep the contrast set to -2 on my 20D, and then any white highlight (with the correct WB setting) will clip the histogram and image just before it clips the green channel of the RAW data. For colored highlights, it gets very tricky. Signature<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<> John P Sheehy <JPS@no.komm> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>>< Thanks John - you make my head hurt, but I think it's all for a good cause! Cheers, Colin >Thanks John - you make my head hurt, but I think it's all for a good cause! No pain; no gain. "Simple & easy to swallow" is usually wrong. Signature<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<> John P Sheehy <JPS@no.komm> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>>< > No pain; no gain. I know, but with me it's usually a case of "getting to grips with it - having lunch - then having to start all over again"! Seriously, it does help - at least now I'm still slow, but I'm starting to make better choices in the field, and definately getting a higher % of keepers because of it. As far as ISO selections go - for the most part, it's not difficult in that in most cases it's dictated by needing a certain aperture for DOF control, and a minimum shutter speed to avoid shake (even with an IS lens). I do find the grain of higher ISOs disappointing, but recently discovered that I can run noise reduction filters over it then sharpen the sh.t out of it, and still end up with something that looks quite reasonable up to about 8 x 12". I still need to learn more about exposure adjustments (I mostly use partial metering, with the subject in centre frame). Cheers, Colin >I do find the grain of higher ISOs disappointing, but recently discovered >that I can run noise reduction filters over it then sharpen the sh.t out of >it, and still end up with something that looks quite reasonable up to about >8 x 12". To each his own - I generally don't do any noise reduction, because even the best software removes detail as well. Noise is most visible in large prints, and 100% computer views (or downsizes with the brutal "nearest neighbor" algorithm, which actually increase visible noise). >I still need to learn more about exposure adjustments (I mostly use partial >metering, with the subject in centre frame). When I use auto-exposure modes, I set EC based on contrast (I use evaluative metering). Small white objects might make me drop the EC down to -1/3, like egrets or gulls that are small in a scene (or the sunlit parts of them are small), or sparse white clothing. Same for dappled light, sun and mostly shade, about +1/3 to +1 for medium contrast scenes, and up to +2 or even higher (in manual) for low-contrast scenes. If I elect to shoot at ISO 800 and +2 EC in heavy fog, I will wind up with a much higher quality recording which can have its contrast boosted more in post processing than if I shot at ISO 200 with 0 EC. Of course, I also shoot a lot in conditions where + EC is impossible; ISO 1600 grossly under-exposed. Colors are not perfect; lots of noise; but these shots are only possible in this kind of light. Almost three stops under-exposed at ISO 1600 (and already with a compromised shutter speed): http://www.pbase.com/jps_photo/image/54006354 Signature<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<> John P Sheehy <JPS@no.komm> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>>< >>I do find the grain of higher ISOs disappointing, but recently discovered >>that I can run noise reduction filters over it then sharpen the sh.t out of [quoted text clipped - 5 lines] >large prints, and 100% computer views (or downsizes with the brutal >"nearest neighbor" algorithm, which actually increase visible noise). You can control the areas where noise is removed by using software such as Noise Ninja's brush function. ********************************************************* "I have been a witness, and these pictures are my testimony. The events I have recorded should not be forgotten and must not be repeated." -James Nachtwey- http://www.jamesnachtwey.com/ >"If I'm struggling to get a shot in poor light - in terms of noise - I'm >always better off to use a a higher ISO than to have an image more than a >stop under exposed at 1/2 that ISO"? One common question I see at DPReview is "why is the sky so noisy in my ISO 100 picture?". The sky is noisy because it dominated the camera's metering, and wound up at a dark midtone range of exposure at ISO 200, and the red channel. being much weaker than the blue or green, is even more poorly exposed, and is posterized. If the sky is the brightest thing, and composes most of the frame, then +2 or more EC was called for, and while the best alternative is to do +2 EC at ISO 100, if it is not possible, +2 at ISO 400 is still better than 0EC at ISO 100. Another way to look at it is that high-key exposures don't vary much in quality between ISOs; not the way the midtones and brighter shadows do. Can you tell if the top or bottom is ISO 1600, vs 100, without reading the caption?: http://www.pbase.com/jps_photo/image/46173328 Signature<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<> John P Sheehy <JPS@no.komm> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>>< >>"If I'm struggling to get a shot in poor light - in terms of noise - I'm >>always better off to use a a higher ISO than to have an image more than a [quoted text clipped - 4 lines] > for, and while the best alternative is to do +2 EC at ISO 100, if it is > not possible, +2 at ISO 400 is still better than 0EC at ISO 100. If there aren't any bright whites to be blown out, you should overexpose to get the image as light as possible, even if that means pushing the ISO. Then tone it down in post processing. >Why is there an ISO setting for a dslr? I always set my ISO setting for tha >appropriate film I used, but without film, why bother with ISO? It defines the sensitivity. More importantly: so that you can have the shutter (motion freezing or blurring) and depth of field you require (differential focus) without blowing highlights or loosing too much in the shadows; unlike film where you have to decide before you load-up and go out, with dSLR you can vary it from frame-to-frame. I do wish my dSLR would let me select and adjust ISO setting in the same way I change shutter and aperture. SignatureIan G8ILZ > Why is there an ISO setting for a dslr? I always set my ISO setting for tha > appropriate film I used, but without film, why bother with ISO? For the same reason that you chose the ISO of your film. As anyone using an film camera ought to know, higher ISO numbers mean greater sensitivity to light, but also usually mean some loss of sharpness. So you choose a particular ISO of film to best suit the lighting conditions under which you'll be shooting. The ISO setting on a film camera is simply to tell the camera the ISO rating of the film you're using, which allows the camera to adjust automatic settings to the film. And quite a few SLRs don't actually have an ISO setting; they automatically read the film cartridge, and adjust themselves. The ISO setting on a DSLR doesn't serve the same purpose; instead, it's the digital equivalent of choosing the ISO of your film. Same as with film, setting a higher ISO gives higher sensitivity to light, but you'll lose some sharpness. IOW, in circumstances where I would load my film camera with 200-speed film, I'll set the DSLR's ISO to 200. If I find that doesn't give me a fast enough shutter speed (I'm usually shooting high-speed dog sports, and flash is prohibited), I'll up the ISO, knowing my pictures will be a bit grainer but I won't get motion blur. The beauty of the DSLR, in that respect, is that you can change ISO from frame to frame as shooting conditions warrant, whereas with a film camera you have to change the entire roll of film. >Why is there an ISO setting for a dslr? I always set my ISO setting for tha >appropriate film I used, but without film, why bother with ISO? > >Thanks in advance. Make this simple, in film U have ASA (ISO) Higher the number the more grain, but film is more sensitive to light. Low light use a higher number but U will get more grain. Same is true with digital but U call it noise, not grain. In digital the higher the ISO the more sensitive the sensor is to light. The same rules apply to both. nothing new. OK technically noise can be added from a few other issues but this is why U have ISO in digital. 400 ISO looks great on the 20d I just received the 5d which I will be checking out. reheien@sbcglobal.net |
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