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Comment by drblast | Hacker News Reader

Can anyone explain why the "ISO" setting exists on digital SLR cameras? It seems like this is an unnecessary holdover from film cameras.

I ask this as an owner of a Canon Digital Rebel that knows very little about photography. What does changing the ISO setting get you that changing the exposure time or aperture wouldn't?

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ISO is sensor sensitivity. Higher settings multiply the numbers that come from individual sensor elements by increasingly large amounts.

Image sensors are not perfectly noiseless, of course, and the higher the ISO is, the noisier the image, which appears as random speckles of colored dots.

You don't notice the difference, though, because you're probably shooting in JPEG mode, not in RAW. In JPEG, the Rebel tries very hard to cover up its sins, and will blur away sensor noise, which is hard to notice unless you grovel over areas of fine detail at 400% magnification.

Second time I've linked this in this thread, but http://www.dpreview.com/reviews/nikond5100/page20.asp shows some great examples of what sensor noise looks like over a range of ISO values.

Edit:

Useless editorializing: A SLR isn't a great purchase for someone who knows very little about photography. A point-and-shoot is lighter, cheaper, more durable, easier to use, and in the vast majority of situations, will produce identical results to a SLR. You then graduate to a real camera when get fed up with the various fiddly technical limitations of a point-and-shoot; and bonus, you still have the small camera, which is the one you take with you when climbing mountains, or on vacation, rather than lugging around five kilograms of japanese glass.

Tedious polemic: I dearly hope you have more than one lens. The enormous price premium of a SLR is mostly for the privilege of being able to change lenses. If you just stick with the kit lens, then you will have spent rather a lot of money to accomplish nothing a point-and-shoot couldn't do.

Just as a counterpoint (not to pick a fight), I'd disagree with a lot of what you said in the latter portion of your post.

Entry level SLRs are great, even for people who don't know anything more than point-and-shoot. The advantage of an SLR is that it will focus faster, meter faster, and ultimately take the photo faster. This is a huge advantage for parents, whose children are fast moving targets.

What's not necessarily a good idea, is buying the biggest, most expensive SLR you can find. Canon and Nikon both make entry-level SLRs that have modes specifically targeted at novice photographers who will never leave full-auto mode. Buying a more expensive model primarily gets you features. Features that you won't use. The hardware is remarkably similar until you get up in to cameras that cost thousands of dollars.

Many people believe that the huge lens is what makes SLR photos clearer in more conditions, but that's not the whole story. The lens is large because of an aspect of the SLR that you'll probably never see: the sensor. The single greatest reason for the increased image quality of an SLR is sensor size. When you shoot with a compact point-and-shoot, you're capturing light with a sensor the size of your pinky fingernail. You could shoehorn the fastest, largest, most expensive SLR lens on to a compact point-and-shoot, and the photos it takes wouldn't compare to an SLR with even a modest lens. The sensor in an SLR camera is closer to the size of a postage stamp, and this makes all the difference.

Keep in mind that compacts these days shoot comparable resolution (megapixels) to SLRs. This means that each "pixel" in a compact is microscopically small when compared to the "pixels" in a SLR sensor. This means less area for light to fall, and less light collected.

So, even with one lens, and in the hands of a novice, a reasonably priced SLR can result in sharper, more well timed photos in a more diverse range of conditions.

Also not to pick a fight, especially since your post has a lot of merit if this were 2009, but point and shoots and bridge cameras have come a long way. You can essentially get all the benefits of an SLR in the higher end point and shoots for several hundred dollars less than even entry level SLR kits. There are a number of point and shoots with the same size sensors as entry level DSLRs, same instant on, equivalent lack of shutter lag, equivalent metering, etc. etc.

For most people, the upsell to the DSLR probably isn't worth it at this point.

There's a tradeoff between the physical size of your lens, the physical size of your sensor, and your maximum optical zoom. The way you put it, you make it sound like you can have it all in a compact, but that's just not the case. A bridge camera with a substantial zoom and large sensor size will be as about as big as a DSLR with a lens of similar capability. A compact point and shoot will either have a smaller sensor or a less capable optical zoom, and probably both. I have a fairly high end compact, albeit one from late 2009 (Canon S90), and it is nowhere near as capable as an entry-level DSLR in terms of focus speed, continuous shot rate, instant on etc.; and the S95 from late 2010 isn't much more capable (720 HD video, stereo mics and firmware update).

imho the 2 biggest differences between a DSLR and a compact is the focus speed and the dynamic range. DLSRs have larger sensors, so by laws of physics, they gather more light and will always have more dynamic range than compacts or smartphones

Mostly yes, but there are a number of compact cameras that now have aps-c sized sensors; the same ones used in entry level dslrs.

I can think of four off the top of my head. Sigma DP1, sigma DP2, Leica X1, and the Fuji X100. The first two will cost you as much as an entry level DSLR, the latter two a lot more. Non of them can be considered a reasonable equivalent or replacement for an entry level dslr. What cameras where you thinking about?

I think he refers to the new mirrorless cameras, such as panasonic gf1 or sony NEX system. but they do have interchangeable lenses

Well of course not. You also don't get interchangeable lenses. I never said you could get it all in a compact.

My point was, for most people (particularly those who this neat little program was made for), the quality in the high end point and shoots is there. The G12 I used late last year performed very well in all of the categories you mentioned. And for nearly half the price of a basic dslr kit, by the time you're all done with it.

http://www.imaging-resource.com/PRODS/G12/G12A6.HTM indicates that G12 power on to first shot is in the region of 2.7 seconds. I can turn on my D90 and take a shot in a single continuous move of picking it up and aiming it - http://reviews.cnet.com/digital-cameras/nikon-d90-with-18/45... says it's 0.2 seconds, though with AF added in, I'd say it feels more like 0.9. Somewhere between 15x and 3x faster makes a huge difference in reacting quickly in e.g. a party, or sport. The D90 gets over 4 shots per second continuously, but it seems that the G12 has a hard time getting 2. The sensor is also pretty small (AFAIK it's the same as the one in G11 and S90); that lets it get higher zoom levels at the cost of quality in low light. All that in a pretty bulky package (IMO - it's bigger than the S90, nearly twice the weight, and the S90 isn't exactly small).

What I'm getting at here is that if you're walking about on holiday in a city, something like the S90, or maybe the G11, would make sense (though I think the G11 might be a little too big for comfort, the S90 is already quite a bit). If you're specifically trying to take photos, rather than just bringing a camera along, then you have a lot more freedom with a DSLR.

My point is that, for people who don't know how aperture, ss, and iso affect exposure, the 2.5 second difference in startup time and the 2 fps difference in burst probably isn't going to make a big difference in their user experience.

Of course you're going to have more freedom with a DSLR. That's the whole point. But, if you can't figure out what the aperture dial does, the freedom is probably going to be lost on you, along with the extra $400 you're going to spend.

I'm not anti-DSLR or anything. I've probably bought and sold $50k worth of gear in the last 5 years. DSLRs are awesome and they keep getting better. But, compact cameras have gotten pretty damn good in the last couple of years. For many people, and many different applications, they will fit the bill. If you're a sports shooter, they certainly won't.

I feel like you're getting piled on, sorry. Although I guess someone would have come along and made the point.

I don't disagree with you entirely. There are APS-C compacts available that have many of the advantages of a DSLR, but I can't see where buying something like a G12 for $500 makes sense when a Nikon D3000 is available at the same price point.

My opinion is that if you need a compact, get a compact. Pick up a Canon SD1300 for $150 and go crazy. I did a (personal, not school) photography project with my SD870IS and took some of the best photographs of my life.

However, I feel that parents, especially, can benefit from a DSLR, specifically because of the speed. My mother bought a Nikon DSLR a couple years ago at my recommendation because she complained of "always missing the shot she wanted" with her Canon compact. She's been thrilled with her DSLR because she can pick it up, flip it on, and start shooting. There's no waiting for a screen to come on, she simply flips the switch that is right under her index finger, looks through the viewfinder, and pulls the trigger. When your 4 year old grandson is doing something adorable in the living room, the difference between compact speed and DSLR speed does make a difference.

I see your point completely. I will say that the best camera is the one you have with you. The reason to buy a G12 over a d3000 is because you can literally fit the G12 in a pocket (although not the most comfortable solution) or in even a small purse. You can't with a d3000. It's something you have to always consider "Do I want to take this with me?"

I was not clear above in the discussion of image quality (got sidetracked with more or less irrelevant arguments about sensor size/dynamic range/etc.). I was attempting to say that image quality as a whole from the higher end point and shoots is on par with DSLRs for almost everyone. See here for a comparison with the 2 generation old G10 v. a $40k medium format rig: http://www.luminous-landscape.com/reviews/kidding.shtml

Looks like we agree on a lot :)

I take up photography in stints of a few weeks to a few months. I get inspired to do some photography, take some photos, then leave it for other pursuits. The last time I took it up, my gut reaction was that I needed to replace my old Canon A1 with a modern DSLR. Prices had come down significantly, and I couldn't possibly do good work with my compact, right?

Well, rather than rushing out and buying a DSLR, I decided to go shoot with my compact SD870IS. I'm so glad I did. It gave me a chance to re-focus on the basics without worrying about what settings I was using. I left the camera in full auto and focused instead on LOOKING at what I was about to shoot. I was really pleased with the result, and I'm 100% convinced that it improved my photography as a result.

Some examples of the photos I shot with the SD870IS:

http://www.flickr.com/photos/bradleyland/3224710694/in/photo...

http://www.flickr.com/photos/bradleyland/3224711584/in/photo...

http://www.flickr.com/photos/bradleyland/3223853289/in/photo...

http://www.flickr.com/photos/bradleyland/3224779424/in/photo...

I could not have taken the shot of the grasshopper with a DSLR because A) I wouldn't have been able to grab the camera, mount a macro lens, and get ready to shoot in time, and B) the DSLR wouldn't have fit in the space I had to put the camera to get that shot.

Then again, I couldn't have taken these shots with the compact because I couldn't quickly change settings for good DOF (first photo) and I'd have missed the moment because of speed (arguably for both).

http://www.flickr.com/photos/bradleyland/3296469446/in/photo...

http://www.flickr.com/photos/bradleyland/3241790710/in/photo...

I just checked BHphoto and the G12 costs more than an entry level Canon,Nikon or Sony dslr with a kit lens. So while the G12 is a great camera (I have a G10) and there are many good reasons to buy on, price isn't one of them

Yes, they're about the same price. Once you kit out the DSLR with a lens that covers a comparable range as the pocket cam, you're at a couple hundred more. Plus you've got to carry two (possibly three) lenses around.

We bought the camera in 2005. The quality of entry-level digital cameras was horrendous at that time. The primary concern was that it would take pictures without lag, so you're exactly right.

ISO is certainly not an unnecessary holdover from film cameras. It's actually very, very useful.

To understand, you really need to get a grasp on the basics of exposure. This may be helpful: http://www.ajkesslerblog.com/understanding-exposure/

When a photon hits a pixel it produces an electrical signal. You can think of the ISO as the amount that signal is amplified. So, if you want to keep aperture and shutter speed at certain levels, you can adjust the ISO to produce a usable image. The tradeoff is that when you amplify a signal a great deal, it gets noisy.

So, does that make the ISO setting a no-op in RAW mode? It sounds like it's entirely a post-processing option.

Changing the ISO at time of exposure actually increase the amplification at the sensor level. Changing the exposure in post processing just shifts bits around (the advantage of RAW is that, depending on the camera, it captures between 12-16 bits vs. 8bits for jpeg images and it's not compressed so shifting the bits around doesn't have quite as noticeable an effect).

Depends on the camera. On medium format digital backs it is, as well as the Nikon D7000.

ISO is a trade-off between noise and speed. Sometimes you need the speed, like at a nighttime party or sports, and sometimes you need the perfect details, like shooting landscapes.

In general, you want to shoot at the lowest ISO which works in any given situation.

A longer exposure can result in object or camera shake blur. Wider aperture reduces depth of field, making it harder to focus.

Sometimes, you'll need all three to get enough light in the shot: A hot sensor, a longish exposure, and a wide aperture.

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