Highlight Tone Priority + ISO question

[rubo]

<p>

 

<p>I have an XSi and when turning on the HTP the ISO100 is not available, the lowest ISO is 200.<br>

Is there really much to gain from using HTP @ ISO 200 compared to just using ISO100 without HTP?</p>

<p>I never liked ISO 200 films, just didn't see much benefit over shooting @ 100 (some times i would just push the 100 to 200 or even 400).<br>

In general i like shooting the lowest ISO possible, unfortunately ISO less than 100 is not available on XSi at all, but that's what i get for wanting a light and small body :)</p>

<p>I guess this sounds like rambling , but really would like to hear from everyone their take on this.</p>

<p>Thank you in advance.</p>

 

</p>

[PuppyDigs]

<p>The main gains from HTL are increased noise in mids and shadows. I leave it off. If you need more HL range, shoot RAW and do a little extra processing in PS layers. Thus you can keep clean shadows.</p>

[rubo]

<p>So it does introduce more noise in the shadows. That's what i thought :)</p>

[zml]

<blockquote>

<p>In general i like shooting the lowest ISO possible, unfortunately ISO less than 100 is not available on XSi at all</p>

</blockquote>

<p>I suppose this is a holdover from the film era but digital works in mysterious ways...<br>

You may want to take a look for instance <a href="http://www.dxomark.com/"><strong>here</strong> </a>for some data re: ISO vs. dynamic range in digital and start exploring the subject - it ain't film, you know... As for the HTP issue, try it on a suitable subject (groom and bride is a cliche here) even if you shoot RAW (it is performed on the sensor level) and form your own judgement. You may also take a look <strong><a href="http://www.canon.co.jp/imaging/picturestyle/file/index.html">here</a></strong>: custom picture styles (as a starting point for conversion if you use RAW) work very well in many situations when you need to preserve highlight details HTP or not.</p>

[phule]

<p>You should compare the results of straight up ISO 100 and 200 shots (not HTP). I don't think you'll find a difference in quality.</p>

<p>Puppy Face echos what dpreview found:</p>

<p>http://www.dpreview.com/reviews/canoneos450d/page22.asp</p>

<p><em>"Although it can save a shot that's been inadvertently over exposed you're far better off shooting raw and pulling back the exposure later."</em></p>

 

[chrisjb]

<p>HTP works quite well up to a point, it seems to cover one stop, blowing highlights more than a stop will cause more probs, its just another tool to be used when needed. I find for some reason 200ISO does give a slightly better result such as in colour, because of workflow generally only use jpeg. Pushing 100ASA film to200~400 was popular for changing the contrast properties of film, my fav result was with 400 portra at 320ASA. Digital is totally different and at 800ISO for most work I get better results than ever with film. Spend time and explore, still, depends on your photo types. mines mainly nightwork with people and flash :)...just my take...cheers</p>

[chrisjb]

<p>BTW, someone may correct me, but in simple term, native ISO of the canon sensor is approx 100 ISO. when you increase ISO you amplify the signal, bit like the ol plastic record, you can`t decrease the volumn of the record so when you decrease ISO under 100 then you lose quality such as DR range. best to read a lil to work it out :0)</p>

[mcgovernphoto]

<p>When you enable Highlight Tone Priority, the camera's computer switches the minimum ISO to 200 and uses that for the majority of the photo. When it processes the image, it isolates the highlights and applies an ISO 100 effect to those areas. That's why you can't manually switch to 100 when HTP is enabled. There has to be a lower ISO to which to apply the highlight area(s).<br>

I don't think that in most cases most people will be able to tell with the naked eye the difference between ISO 100 and 200 in the final image. ISO 200 on the XSi does not introduce a noticable amount of noise.</p>

[rubo]

<p>Thanks for the info, much appretiate it.<br>

I'll need to do some more tests (under extreme lighting) to see if i can find any difference in noise between ISO 100 and HTP.</p>

<p>According to all the websites the XSi sensor is actually worse in respect to DR, sensitivity and noise compared to my old XTi, but i find the pictures out of XSi to be better (maybe it's just me).<br>

But the 12MP is really pushing the limit of the sensor in terms of noise and diffraction blur.</p>

<p>Yeah Michael, i used to love using ISO 25 and 50 slide film back in late 90's.<br>

I find it quite easy to make the shots look like those fils in PP, but still miss using them :)</p>

<p>Thanks again and keep'm comming.</p>

[allardk]

<p>As far as I've understood HTP underexposes one stop by lowering the ISO amplifier one step, and then partially corrects for this digitally with a tone curve that corrects more for the shadows than the highlights. You can do this yourself in RAW by shooting at ISO 100 and underexposing one stop, and fix the exposure in PP, but you don't need to do that unless you don't like the look HTP gives you.</p>

<p>There may be some more tricks in the image processing of HTP, see a recent DPreview article on the company that sells a special algorithm for this.</p>

[steve_dunn2]

<cite>BTW, someone may correct me, but in simple term, native ISO of the canon sensor is approx 100 ISO. when you increase ISO you amplify the signal, bit like the ol plastic record, you can`t decrease the volumn of the record so when you decrease ISO under 100 then you lose quality such as DR range.</cite>

 

<p>Correct. A few EOS DSLRs offer ISO 50, but it costs dynamic range, because it's actually implemented as ISO 100, overexposed one stop (so highlights wash out a stop sooner than at 100), then pulled back down a stop after the analog-to-digital conversion (equivalent to setting -1 compensation in your RAW converter). This is why, like the higher ISOs (H, H1, H2, depending on what body you have), it's only available if you enable it via a custom function: this is Canon's way of saying "We don't really recommend doing this unless you know what you're doing."</p>

 

<cite>As far as I've understood HTP underexposes one stop by lowering the ISO amplifier one step, and then partially corrects for this digitally with a tone curve that corrects more for the shadows than the highlights.</cite>

 

<p>I don't believe Canon has ever stated exactly what is done; they would classify this as a trade secret. But the loss of ISO 100 strongly suggests that they're underexposing, which is logical since it would increase headroom for highlights (and the observations that there's more shadow noise in HTP also suggest the use of a higher ISO). And to make good use of that extra headroom would require a differently-shaped tone curve.</p>

 

<p>So yes, you could achieve the same end by shooting RAW, underexposing, and then playing with the parameters in your favourite RAW converter. You'd probably end up with at least a slightly different look, since you'd probably come up with a slightly different tone curve than what Canon uses; whether this is good or bad depends on how appropriate the various curves are to your eye and/or subject matter. And as with so many other things, you have the opportunity to trade off less work (let the camera do it for you) versus more control (make your own tone curve in your RAW converter).</p>

[davebell]

<p><cite>BTW, someone may correct me, but in simple term, native ISO of the canon sensor is approx 100 ISO. when you increase ISO you amplify the signal, bit like the ol plastic record, you can`t decrease the volumn of the record so when you decrease ISO under 100 then you lose quality such as DR range.</cite></p>

<p>In layman's terms you're right. With my pedantic electronic engineer's hat on, no. I would money on it that there is definitely amplification even at ISO100.</p>

[keith reeder]

<blockquote>

<p>But the loss of ISO 100 strongly suggests that they're underexposing</p>

 

</blockquote>

<p>I can guarantee it.</p>

<p>Not long after first getting my 40D (back in September 2007) I set HTP while getting familiar with the camera's features, then forgot about it and went for a day's shooting of Winter waders (shorebirds).</p>

<p>I then converted the images in a converter <em>other than DPP</em> and - sure enough - every last one was about a stop underexposed, even though the camera settings (HTP apart) were spot-on for the shooting situation.</p>

<p>Sticking 'em through DPP, the same images were fine, exposure-wise - if a bit noisy...</p>

[rubo]

<p>

 

<p>I did some tests last night and the findings were as follows:</p>

<p>1. With ISO 100 same shot had blown highlights showing up on the histogram view until about 2/3 stop underexposed.<br>

2. With ISO 200 - same results.<br>

3. With HTP enabled (ISO 200) with proper exposure - no blown highlights. The shot did seem a very little underexposed, but it wasn't even 1/3 of a stop.</p>

<p>As far as noise, i could see some very minimal increase in noise after going from ISO 100 to ISO 200, but it was very negligible.<br>

HTP didn't add any more noise compared to ISO 200 with it off.</p>

<p>The conclusion that i came too (and this is only MHO) if you shoot very high contras subject and don't plan on printing very, very big prints use the HTP. Otherwise just use ISO 100 and expose the shot properly :)</p>

<p>Thanks for all the replies. As always this is a great place to learn :)</p>

 

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[tracy_king1]

<p>I was playing around with HTP once a couple weeks ago taking pictures of my baby and figured it would be a good thing to try being that she has a nice little bright helmet to (to reshape her head, if your curious look up plagiocephaly) and it is very prone to getting overblow compared to the rest of her when using my flash's. I noticed though that when I enabled it sure the helmet didn't get overblown as easily but I also noticed the pictures just seemed to loose their life and pop as well. this is just straight JPG comparison here but with HTP setup the pictures just seemed a bit more bland and I quickly switched it back as the overblown spot was a lot more livable to me in this situation.</p>