Field Curvature vs Pincushion (1 Viewer)

Please excuse my lack of experience, but after reading a couple of the recent reviews I am a little confused. I thought that field curvature, pincushion distortion and edge softness were pretty much the same thing. Can anyone explain the differences and tell me how I could tell one from the other?

Thanks for any help.
Rene

Rene,

I guess we've been standing back waiting for the experts on the forum to post a definitive answer. In the mean time I'll have a go. There is a basic explanation here:
http://www.monkoptics.co.uk/General/about-binoculars.html

Generally binoculars have a sharper focus in the centre on the field than at the edges. How rapidly the image deteriorates is an indicator of optical quality. Some designs may narrow the field of view to mask the deterioration, but typically sharp edges are expensive. However, this is easily confused with field curvature. In practice it means that the focal point at the edges of the field is closer than at the centre. Whether this is good or bad depends on personal preference. Those who like it might describe the view as having a greater depth of field, or a 3-dimensional view, though both are totally incorrect descriptions in this context. Those who don't like it may say the edges are fuzzy, hence the confusion. Many designs now have a field flattening or aspheric lens which will reduce the field curvature. So if you are standing square or to a target like a flat sided building then more (or all) of the view will be in focus. Getting this right without introducing other distortions is tricky and expensive. Most binoculars will have a certain amount of pincushion built into the design (check the link). The benefit is that when you move the binoculars from side to side the view appears more stable. Too much or too little can be nauseating for some, but not everyone notices it.

The important thing is to try a lot of pairs and buy the one that you enjoy using. Trying to figure out why you like it can wait 'till much later. Most of us are still trying to work that out and is the subject of much debate here.

Hope this helps. I'm sure the experts will pick up on any errors of mine or add more detail if required.

David

Just looking at the image for barrel distortion (rolling ball I assume) makes me a little dizzy. The pincushion I can deal with.

Field curvature can be focused out. That is one way to distinguish that.

They're three different things: two fo them affect how sharp things are at the edge of field.

Field curvature is literally the focal plane of a lens(es) is not flat. As Steve says you can change the focus to focus out the field curvature. You can also "accommodate" out" the field curavture (depending upon how much accommodation your eyes have) so different people especially at different ages will see different amounts of field curvature.

"Edge softness" can include astigmatism and coma. Astigmatism occurs when the focal point varies depending upon the orientation of the line in the field: usually measured along the radius of the field (saggital) and at right angles to a radius (tangential). Both are aberrations that increase to the edge of the field.

Pincushion distortion is added to reduce "rolling globe or rolling ball" effect. It's a feature not a problem though what amount is the "correct" amount varies with the user.

http://www.birdforum.net/showthread.php?t=161164
http://www.birdforum.net/showthread.php?t=74727
http://www.birdforum.net/showthread.php?t=140202