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whenever i take a quick photo and i need to straightening in post it never seems quite the same as if i shot it with a straight horizon to begin with. (especially with a busy photo with a lot going on)

 

besides the loss of pixels from straightening the image, is the image the same as if i shot it straight to begin with, or is there something else going on here?

 

not sure if my mind is going crazy.

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Nor I. Straightening is just a rotation and crop. It does not change the perspective at all, unless you go for the horizontal and vertical perspective corrections in Photoshop.

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... it never seems quite the same as if i shot it with a straight horizon to begin with.

 

How do you know?...do you take a second pic with a straight horizon to compare? Of course you don't (although you might try); the point is that different is not the same, as one wise photo teacher used to say. Once you edit a photo, it's not the same as the unedited version (certain elements in a complex 'straightened' photo are now rotated closer to the picture edge, etc. etc.), and the scene you initially saw is only a memory in your mind.

 

Jeff

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I agree that straightening a photo can make it look different simply because it imposes a slight crop, so changing the effective focal length of the lens. I think with wider lenses this can feel more pronounced because things at the edge of the frame often have a more dynamic relationship to the image as a whole, and losing them can dilute the sense of drama. On the other hand straightening up a portrait (so the lampost sticking out of the head is vertical :D) has less of an impact because things on the edge of the frame will not generally be a vitally important part of the image.

 

For me I most often need to straighten the horizon when using a wide lens, but almost never with a longer lens.

 

Steve

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Whenever I take a quick photo and need to straighten in post-processing, it never seems quite the same as if I shot it with a straight horizon to begin with. (especially with a busy photo with a lot going on).

 

Besides the loss of pixels from straightening the image, is the image the same as if I shot it straight to begin with, or is there something else going on here?

There is something else going on indeed. After the rotation, no pixel will be the same as it previously was. All pixels in the straightened image will be recomputed from the original pixels; they are not simply relocated or remapped. This means that very fine detail may get softened or even lost ... at least in theory.

 

That's why straightening is one of the first operations to do. In particular, never rotate a picture after downsizing! Rotate first, then downsize (if required). You may want to apply some mild capture sharpening before rotating, in order to preserve more of the finest detail. After rotating, capture-sharpen again. In most cases, however, the loss of detail through image rotation will be negligible, so don't be obsessed about avoiding it! By the way, it does not matter how far the image gets rotated ... be it 0.1°, 1°, or 10°—in either case, all original pixels will get lost, and the image will be built up from all new pixels, computed from the old ones.

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Olaf is correct ...... and I was roundly condemmed for suggesting there is a loss of sharpness in several threads in the past. Only multiples of 90 deg preserve image quality ..... all other rotations result in interpolation and some loss of clarity ... and in fact it seems most marked with small angle rotations.

 

It is very obvious in Aperture on images with a lot of fine detail ..... LR makes a much better job of it though ....... but it is never without some small effect

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But it's not just about the 'new' pixels, as I mentioned above. Subjects in the photo, especially in a 'busy' photo as the OP describes, i.e., one with multiple elements around the frame, will be repositioned. If the horizon is tilted down toward the right for straightening, for instance, elements in the bottom right of the pic will now be closer to the lower frame edge than before.

 

Even small changes near the frame edge can often make a critical difference in the photo composition. Check some Paul Strand photos to see what I mean.

 

Jeff

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I think you can straighten an image in Photoshop without worrying a lot about the loss of sharpness.

 

It is now a very sophisticated algorithm used and many of the older ideas of image degradation have been superceeded, just as noise algorithm's have improved in ACR to the point that separate noise reducing software isn't generally needed, just as interpolation algorithm's have improved so you don't need specialist software to add pixels, just as sharpening is now a league away from what it was in Photoshop a few generations ago.

 

Steve

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There is something else going on indeed. After the rotation, no pixel will be the same as it previously was. All pixels in the straightened image will be recomputed from the original pixels; they are not simply relocated or remapped. This means that very fine detail may get softened or even lost ... at least in theory.

 

That's why straightening is one of the first operations to do. In particular, never rotate a picture after downsizing! Rotate first, then downsize (if required). You may want to apply some mild capture sharpening before rotating, in order to preserve more of the finest detail. After rotating, capture-sharpen again. In most cases, however, the loss of detail through image rotation will be negligible, so don't be obsessed about avoiding it! By the way, it does not matter how far the image gets rotated ... be it 0.1°, 1°, or 10°—in either case, all original pixels will get lost, and the image will be built up from all new pixels, computed from the old ones.

 

My understanding is that LR will do this in the correct order, not sure about photoshop. The other consideration with pixel mapping is how much you rotate, a very small amount will be very hard in terms of this mapping and I suspect the softening/jaggies or artefacts will be at there worst at some positions. Having said that perhaps LR, Photoshop simply stop you positioning them at these tricky positions ?

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