I've no experience of the 135-400mm but I do have the 50-500mm which I bought for a safari trip last year and intended to sell on. However it's such a useful lens with that wide zoom ratio that I kept it and use it often, most recently at the Margate air show where others using 80-400s were struggling a little on distant shots and losing out altogether as the planes came close. Using continuous focus and keeping my finger on the AF button I unintentionally cropped a few fast jet shots because of my inability to stay with the action but every single shot of the 11 gigs worth of raw images was sharply focused - a testament to my D200 and Sigma's focusing performance. As regards deer shots at Richmond I take, but rarely use the "Bigma". I find I can easily get close enough to use my 18-200VR even in the rut which quite frankly is the only time deer are worth photographing IMO. As regards bird photography, you need to check out the closest focusing distance. My Bigma needs to be at its closest focus of eight feet to get a frame filling image of something as large as a Greater spotted woodpecker and so is usually mounted on a tripod and fired by wireless remote. As far as IQ is concerned I think the more restricted zoom ratio of the 135-500 may actually be a good thing and I've yet to be dissatisfied with a Sigma lens. Try a search of the photosites or simply Google "Sigma 135-500mm photos" and you'll find some sites that will show you images taken with this lens and user reviews of the lens itself. If you want to see some Bigma images have a look at http://www.webaperture.com/gallery/g/mb/veejaycee and check out my pics taken with the 50-500mm. Equipment details are shown with each image.