a55 mirror really does hurt high ISO a580 vs a55 vs Pentax

Started Feb 17, 2011 | Discussions thread
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a55 mirror really does hurt high ISO a580 vs a55 vs Pentax
Feb 17, 2011

DPR has not reviewed the a580 (which has the a55 sensor but no mirror), but they have reviewed the Pentax K-5 which has the same sensor.

Using RAW (change to RAW at top of page) so in-camera jpg algorithms are taken out of the mix and compare:

SLT a55 ISO 6400 RAW
Pentax K-5 ISO 6400 RAW
Pentax K-5 ISO 6400 RAW


The coins that automatically come up look reasonably comparable, but it is shadows where the rubber meets the road in high ISO. Slide the selection box to the spools of thread that are in shadow. The SLTa55 is worse than either Pentax. How much worse? Keep the Pentaxes at 6400 and set the a55 to 3200. Now they are pretty comparable, with the a55 having a slight edge.

Conclusion? The translucent mirror has over a half stop penalty, per the DPR test it comes close to a full stop.

Why would this be? We have all heard that the mirror absorbs 1/3 the light. That means 1/3 stop, right? Wrong! A mirror that absorbed 1/2 the light cuts the light in half. That is a full stop. Well, 1/3 is less than 1/2, but not that much less. You would expect about a 2/3 stop hit, and this indicates something near that. Of course we don't really know how much light the mirror absorbs. No one to my knowledge has really tested it. If it absorbs 40% you are awfully near a full stop.

Dxomark backs this up. If you care about high ISO, the a580 should be enough better than the a580 to care about. If you really like the compactness of the SLT and really care about high ISO the Pentax K-X is inexpensive and worth a look as it is quite small.

If you really want good live view the Sonys are best, but the Pentax K-R (same IQ as the K-X) has remarkably solid CDAF in live view, like a good point and shoot, and fast burst.

IMHO the strong point of the SLT cameras is the EVF, because I shoot in low light a lot and the gainup is terrific. An EVF can actually be used to focus in low light! Between the general dimness of pentamirror and the characteristics of laser matte screens I have never seen a pentamirror finder that is good on low light. That is a conundrum for low light shooters. The best finder is mated to a IQ penalty of 3/2 to 1 stop.

I quess the summary is:

Cheap, compact and great high ISO but crummy live view - Pentax K-X
Compact and great high ISO, good live view but not as cheap - Pentax K-X

Best low light finder, compact, good but not great high ISO, great live view - SLT a55.

Best IQ for the money, great high ISO, great live view, not very nice finder - a580

I am not mentioning burst because that is equally complicated, but every camera here has good burst capabilities. Lenses not mentioned either except Sony has the edge unless you have $$$ and don't mind the Pentax limited lens prices.

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