Originally posted by pjv But yes, using the crop mode on your K-1 will give you the field of view of a 450mm lens. Give it a try and see what you think of the results.
The wider point underlying Peter's comment is that when you have lots of megapixels to play with (as you do on the K-1 or the K-3ii), there is plenty of scope for cropping (including using the K-1 in crop mode), so there isn't the same imperative to fill as much of the frame with the subject as there was with a body like the K20D. Using the 55-300 on the K-1 in crop mode is roughly like putting it on a 16 megapixel body (e.g. K-5), except with the benefit of the advances made in the K-1 such as high-ISO performance. That combination may well meet your needs. As Peter says, try it and see first.
Of course if you are trying to photograph a sparrow-size bird with a 300mm lens from 30 metres (say 90 feet) away, the area of the subject's impression on the sensor (so to speak) might be too small to get a high-quality image when cropped hard, even for a shot taken in perfect conditions. There comes a point when there is no real substitute for reach (ie greater magnification), whether using a long lens or a lens with a teleconverter. Where that point is depends on the sensor, the size and distance of the subject, the light, and the quality and reach of the lens.
@WildMark did an interesting comparison to try to answer that question for the K-3 using high-quality lenses and teleconverters in good Australian sunlight:
Teleconverter challenge Sigma vs Kenko vs Tamron X1.4 - PentaxForums.com Of course, change any variable and the result will be different, but it's an interesting general indicator.
Last edited by Des; 09-24-2017 at 02:34 PM.