7d vs 40d exposure question

[mark_englehart]

<p>Hi,<br>

I just got a 7d and was noticing that it seemed to use a bit of a slower shutter speed to what I'm used to with my 40d. I just did a side by side comparison with the same lens, settings, and subject. I was indoors and set the ISO on both to 800. Aperture was F2.8. The subject was a chair. The lighting conditions were the same. The cameras were both set to Av priority with no exposure compensation. The 40d needed a shutter speed of 1/30 sec, while the 7d needed 1/15 sec. Is this because of the higher pixel count? Is this normal? Also it seems to have a harder time focusing in less than ideal lighting situations than the 40d.<br>

Thank you,<br>

Mark</p>

[kari v]

<p>What do the histograms look like? I mean, does the 7D actually need one stop more light to reach the same exposure or is it just different metering?</p>

[john_deerfield]

<p>In other words, was the same metering mode used? If you really want to test this, I would grab a gray card, put both cameras in spot metering, have the gray card fill the frame and see what you get. A correctly exposed gray card will have a histogram "curve" right in the middle of the graph.</p>

[ljwest]

<p>Um, is the shutter "stepless" on our Canons? I know on my older Minolta SLRs, once electronic control of the shutter came along, the usual shutter steps - in automatic modes, at least - went away on most cameras. In manual or Tv modes, the steps remained, though.</p>

<p>So, although the two cameras may show a stop difference in the VF, they may be taking the photo at very similar shutter speeds, and what you see in the VF is merely a rounding difference. If you haven't actually taken the photos, do that, and compare the EXIF data, which, if I recall, will show the exact shutter speed.</p>

<p>As John said, the metering mode could have a lot to do with it as well.</p>

<p>In the end though, does it really matter which speed is selected by which camera, so long as a good photo is the result?</p>

[kari v]

<p>Larry, 1/2 stop should be visible in VF, I guess 1/20 for 7D could be within rounding but full stop is too much.</p>

[mark_englehart]

<p>Thanks for the feedback. I will do a more accurate test when I get home later. The reason I am concerned is that I do a lot of Bar/Bat Mitzvahs as well as weddings and I like to keep the ISO as low as posible. I tend to shoot manual with an off camera flash or 2. Does anyone have insight into why I might be having low light focusing issues?<br>

Mark</p>

[PuppyDigs]

Can you post the images? I owned the 40d prior to the 7d and exposure accuracy is about the same. The 7d seems to

handle tricky side and backlighting a little better but I didn't need to change my basic metering habits one iota. However

most cameras I owned will show different shutter speeds when in AV mode. Different metering algorithms or differences in

actual light transmission of lenses may account for it. Doesn't matter as long as the exposure is accurate. Of if exposure is

not accurate, you can have Canon calibrate the meter.

 

I've generally experienced better low light AF from the 7d. However, it took me weeks to figure which AF modes worked for

me. Auto selection was a total bust in low light. Manually selection or manually selection in clusters worked best for me.

[JDMvW]

<p>No two cameras can be set to take in exactly the same parts of a varied image, never mind different kinds of metering points, systems, etc.<br>

You also need to use the same (the <em>very same</em>, not just another copy of the same model) lens on both cameras, since lens light transmission varies too. If you were using exactly the same lens, OK.<br /> <br /> As said, you need a uniformly lit field to really compare, and even then a 1-stop difference isn't <em>all</em> that much.</p>

[steve_dunn2]

<cite>Um, is the shutter "stepless" on our Canons?</cite>

 

<p>You might describe it as semi-stepless. I don't think Canon has ever officially published specs on it, but the scuttlebutt is that EOS bodies do their metering in increments of something like 1/6 of a stop (which would make sense mathematically since so many of them offer the choice of 1/2 and 1/3 as the increments for setting things like aperture, shutter speed, and exposure compensation). For display purposes, the selected aperture/speed is rounded off to the nearest half or third of a stop (depending on your body and how you've configured it). So the full-stop difference mentioned by the original poster may not actually be a full stop, but it's still a significant difference. I'm not sure if there's anything in the EXIF data that might reveal more precise information about metering than what's displayed on the camera; it might be worth looking at if you're really curious.</p>

 

<p>Do keep in mind that the 7D uses a completely different metering system: different number of metering zones, resulting different placement of the zones, different technology, and in all likelihood adjusted algorithms to work with all of that. It's unlikely that the two cameras will always meter the same number of EVs apart no matter what scene you point them at; it's more likely that the difference will vary from one scene to another, depending on how the metering systems react to it.</p>

[ljwest]

<p>Kari,</p>

<p>You're exactly right. I just wasn't 100% sure. Sorry to add to the confusion!</p>